Concise Bible Dictionary:

Besides the use of this word for any collecting together, “as the assembly of the wicked,” it has a special reference in the Old Testament to the children of Israel as “the assembly,” whether they were collected together or not. Several Hebrew words are used, some implying “an appointed meeting,” others a “calling together.” “The whole assembly of the congregation” were to eat the passover (Ex. 12:66And ye shall keep it up until the fourteenth day of the same month: and the whole assembly of the congregation of Israel shall kill it in the evening. (Exodus 12:6)), though each family ate it in its own house. They accused Moses of having brought them into the wilderness to kill the “whole assembly with hunger,” (Ex. 16:33And the children of Israel said unto them, Would to God we had died by the hand of the Lord in the land of Egypt, when we sat by the flesh pots, and when we did eat bread to the full; for ye have brought us forth into this wilderness, to kill this whole assembly with hunger. (Exodus 16:3)); so in many places. When they were especially called together to the feasts it is often called a SOLEMN ASSEMBLY, as in Leviticus 23:3636Seven days ye shall offer an offering made by fire unto the Lord: on the eighth day shall be an holy convocation unto you; and ye shall offer an offering made by fire unto the Lord: it is a solemn assembly; and ye shall do no servile work therein. (Leviticus 23:36); Numbers 29:3535On the eighth day ye shall have a solemn assembly: ye shall do no servile work therein: (Numbers 29:35); Deuteronomy 16:88Six days thou shalt eat unleavened bread: and on the seventh day shall be a solemn assembly to the Lord thy God: thou shalt do no work therein. (Deuteronomy 16:8); 2 Kings 10:2020And Jehu said, Proclaim a solemn assembly for Baal. And they proclaimed it. (2 Kings 10:20); 2 Chronicles 7:99And in the eighth day they made a solemn assembly: for they kept the dedication of the altar seven days, and the feast seven days. (2 Chronicles 7:9); Nehemiah 8:1818Also day by day, from the first day unto the last day, he read in the book of the law of God. And they kept the feast seven days; and on the eighth day was a solemn assembly, according unto the manner. (Nehemiah 8:18).
In the New Testament the word is also used for any gathering of people, as at the tumult in Ephesus (Acts 19:32,39,4132Some therefore cried one thing, and some another: for the assembly was confused; and the more part knew not wherefore they were come together. (Acts 19:32)
39But if ye inquire any thing concerning other matters, it shall be determined in a lawful assembly. (Acts 19:39)
41And when he had thus spoken, he dismissed the assembly. (Acts 19:41)
). In James 2:2 The word “assembly” is really the synagogue, or place of meeting. In Hebrews 12:2323To the general assembly and church of the firstborn, which are written in heaven, and to God the Judge of all, and to the spirits of just men made perfect, (Hebrews 12:23) the words “GENERAL ASSEMBLY” should be joined to Hebrews 12:2222But ye are come unto mount Sion, and unto the city of the living God, the heavenly Jerusalem, and to an innumerable company of angels, (Hebrews 12:22), reading “and to the innumerable company of angels, the general assembly:” the word “and” dividing the subjects. The Greek word used in Acts 19:3232Some therefore cried one thing, and some another: for the assembly was confused; and the more part knew not wherefore they were come together. (Acts 19:32) is ἑκκλησἰα, and this often occurs in the New Testament where it is translated “church.” It signifies “called out,” and the church is a people called out by God to Himself from the mass of mankind. The church may more accurately be designated by the word “assembly.” See CHURCH.

From Anstey’s Doctrinal Definitions:

The word translated "assembly" (church in the KJV) is “eccelsia” in the Greek. It means “called out ones,” and refers to those who have been called together for a purpose. It is mentioned once in connection with the children of Israel, who were a company of people called out of Egypt into a relationship with Jehovah. While they were in the wilderness on their way to the land of Canaan, Stephen called them “the assembly in the wilderness” (Acts 7:3838This is he, that was in the church in the wilderness with the angel which spake to him in the mount Sina, and with our fathers: who received the lively oracles to give unto us: (Acts 7:38)). The word “assembly” is also used once in connection with a company of heathen Gentiles (unbelievers) who were called together to make a decision regarding their trade (Acts 19:32, 4132Some therefore cried one thing, and some another: for the assembly was confused; and the more part knew not wherefore they were come together. (Acts 19:32)
41And when he had thus spoken, he dismissed the assembly. (Acts 19:41)
Every other reference to the “assembly” in Scripture refers to a special company of persons who have believed the gospel and thus have received the Lord Jesus Christ as their Saviour—i.e. Christians. They have been “called out” from the mass of mankind into a special place of favour and blessing before God in relation to Christ, who is “the Head of the assembly” (Eph. 5:2323For the husband is the head of the wife, even as Christ is the head of the church: and he is the saviour of the body. (Ephesians 5:23)). The term is used in two ways in connection with Christians:
•  Secondly, to describe believers on the Lord Jesus in their local setting (in a town or a city), functioning together as a company assembled for worship and ministry (Matt. 18:1818Verily I say unto you, Whatsoever ye shall bind on earth shall be bound in heaven: and whatsoever ye shall loose on earth shall be loosed in heaven. (Matthew 18:18); Acts 11:22; 13:122Then tidings of these things came unto the ears of the church which was in Jerusalem: and they sent forth Barnabas, that he should go as far as Antioch. (Acts 11:22)
1Now there were in the church that was at Antioch certain prophets and teachers; as Barnabas, and Simeon that was called Niger, and Lucius of Cyrene, and Manaen, which had been brought up with Herod the tetrarch, and Saul. (Acts 13:1)
; Rom. 16:1, 51I commend unto you Phebe our sister, which is a servant of the church which is at Cenchrea: (Romans 16:1)
5Likewise greet the church that is in their house. Salute my wellbeloved Epaenetus, who is the firstfruits of Achaia unto Christ. (Romans 16:5)
; 1 Cor. 1:22Unto the church of God which is at Corinth, to them that are sanctified in Christ Jesus, called to be saints, with all that in every place call upon the name of Jesus Christ our Lord, both theirs and ours: (1 Corinthians 1:2); Col. 4:15-1615Salute the brethren which are in Laodicea, and Nymphas, and the church which is in his house. 16And when this epistle is read among you, cause that it be read also in the church of the Laodiceans; and that ye likewise read the epistle from Laodicea. (Colossians 4:15‑16); 1 Thess. 1:11Paul, and Silvanus, and Timotheus, unto the church of the Thessalonians which is in God the Father and in the Lord Jesus Christ: Grace be unto you, and peace, from God our Father, and the Lord Jesus Christ. (1 Thessalonians 1:1), etc.).
It is noteworthy that Scripture refers to the local aspect of the assembly far more often (about 90 times) than the universal aspect (about 20 times). Distinguishing these two aspects simply requires viewing the context of the passage wherein the term in found.
A common misunderstanding in connection with the local aspect of the assembly is to see it as being merely the sum total of all believers in a particular city or town. But this is not right; it would be making the local assembly to be merely a pared down version of the universal aspect. This faulty definition has led to the idea that there is nothing on earth today in any city or town that can be regarded as the local assembly, because the Church in testimony is in irreparably divided state. However, Scripture indicates that there can still be a local assembly in a city or town, even though all believers in that city or town are not present. The first reference to a local assembly in the Word of God clearly shows that it is those who have been Scripturally gathered to the Lord’s name by the Holy Spirit in a given city (Matt. 18:15-2015Moreover if thy brother shall trespass against thee, go and tell him his fault between thee and him alone: if he shall hear thee, thou hast gained thy brother. 16But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established. 17And if he shall neglect to hear them, tell it unto the church: but if he neglect to hear the church, let him be unto thee as an heathen man and a publican. 18Verily I say unto you, Whatsoever ye shall bind on earth shall be bound in heaven: and whatsoever ye shall loose on earth shall be loosed in heaven. 19Again I say unto you, That if two of you shall agree on earth as touching any thing that they shall ask, it shall be done for them of my Father which is in heaven. 20For where two or three are gathered together in my name, there am I in the midst of them. (Matthew 18:15‑20)). The Lord said that, in attempting to clear up problems that might threaten the unity of the saints, there may come a point when they would need to "tell it to the assembly," and thus apprise them of the difficulty. After speaking of the authority vested in the assembly to act administratively in the matter, if need be (Matt. 18:18-1918Verily I say unto you, Whatsoever ye shall bind on earth shall be bound in heaven: and whatsoever ye shall loose on earth shall be loosed in heaven. 19Again I say unto you, That if two of you shall agree on earth as touching any thing that they shall ask, it shall be done for them of my Father which is in heaven. (Matthew 18:18‑19)), He went on to define what a local assembly is, saying, "For where two or three are gathered together unto My name, there am I in the midst" (Matt. 18:2020For where two or three are gathered together in my name, there am I in the midst of them. (Matthew 18:20)). Clearly then, the local assembly is the saints gathered to His name. Even if it were but two or three so gathered, it would still be "the assembly" in a particular town or city.
When we look at the other references in Scripture that refer to the local assembly, we see that it is something that comes together and functions practically, regardless of whether all Christians in a given locality are there or not. It comes together to break bread (1 Cor. 11:18-2618For first of all, when ye come together in the church, I hear that there be divisions among you; and I partly believe it. 19For there must be also heresies among you, that they which are approved may be made manifest among you. 20When ye come together therefore into one place, this is not to eat the Lord's supper. 21For in eating every one taketh before other his own supper: and one is hungry, and another is drunken. 22What? have ye not houses to eat and to drink in? or despise ye the church of God, and shame them that have not? What shall I say to you? shall I praise you in this? I praise you not. 23For I have received of the Lord that which also I delivered unto you, That the Lord Jesus the same night in which he was betrayed took bread: 24And when he had given thanks, he brake it, and said, Take, eat: this is my body, which is broken for you: this do in remembrance of me. 25After the same manner also he took the cup, when he had supped, saying, This cup is the new testament in my blood: this do ye, as oft as ye drink it, in remembrance of me. 26For as often as ye eat this bread, and drink this cup, ye do show the Lord's death till he come. (1 Corinthians 11:18‑26)) and to be ministered to from the Word of God (1 Cor. 14:3-53But he that prophesieth speaketh unto men to edification, and exhortation, and comfort. 4He that speaketh in an unknown tongue edifieth himself; but he that prophesieth edifieth the church. 5I would that ye all spake with tongues, but rather that ye prophesied: for greater is he that prophesieth than he that speaketh with tongues, except he interpret, that the church may receive edifying. (1 Corinthians 14:3‑5)). Scripture also indicates that the assembly is something that a Christian may not always be in (1 Cor. 14:18-1918I thank my God, I speak with tongues more than ye all: 19Yet in the church I had rather speak five words with my understanding, that by my voice I might teach others also, than ten thousand words in an unknown tongue. (1 Corinthians 14:18‑19)), and something that a person can actually be cast out of! (3 John 1010Wherefore, if I come, I will remember his deeds which he doeth, prating against us with malicious words: and not content therewith, neither doth he himself receive the brethren, and forbiddeth them that would, and casteth them out of the church. (3 John 10)) In fact, unbelievers could even be found in this aspect of the assembly! (1 Cor. 14:23-2423If therefore the whole church be come together into one place, and all speak with tongues, and there come in those that are unlearned, or unbelievers, will they not say that ye are mad? 24But if all prophesy, and there come in one that believeth not, or one unlearned, he is convinced of all, he is judged of all: (1 Corinthians 14:23‑24)) These things show that the local aspect of the assembly is different from the universal aspect. Hence, while the local assembly, in principle, embraces all true believers in a city or town, it may not involve all believers in that locality practically.
J. N. Darby said, "It is clear that the Christians of a certain place, being gathered together, were truly the assembly of that place, but it was not only the assembly that owned God, but that which God owned, and which enjoyed exclusively the privileges He could vouchsafe unto it, as being His assembly" (Collected Writings, vol. 1, p. 260). W. Kelly said, "Where there are but three meeting upon God's principles (that is, church ground), it is, if I may so say, church, if not the church. If there were three thousand real saints meeting, but not on God's principles, they would not be the church" (Lectures on Matthew, p. 327). Hence, while most Christians in a given city or town are not gathered to the Lord's name, those that are on the true ground of the assembly in that town or city are owned of God as such by the presence of Christ in their midst according to Matthew 18:2020For where two or three are gathered together in my name, there am I in the midst of them. (Matthew 18:20).
Having established this point, we hasten to say that it is certainly out of keeping with the whole character of Christianity for those gathered to the Lord's name to formally call themselves "the assembly" of such and such a place. How unfitting it would be in a day of ruin for those so gathered to proclaim that they are the assembly in a particular city or town, even though they may believe that they are truly on that ground morally. J. N. Darby said, "It is clear that if two or three are gathered together, it is an assembly, and if Scripturally assembled, an assembly of God; and if not, what else? If the only one in the place, it is the assembly of God in the place, yet I do object practically to taking the title, because the assembly of God in any place properly embraces all the saints in the one place; and there is a practical danger for souls in assuming the name, as losing sight of the ruin, and setting up to be something. ...but if there will be one such, and another is set up by man's will independent of it, the first only is morally in God's sight the assembly of God, and the other is not at all so, because it is set up in independency of the unity of the body" (Letters of J. N. Darby, vol. 1, p. 424).
As to the time frame of the calling and forming of the assembly in its universal aspect, Scripture indicates that it began with the descent of the Holy Spirit from heaven on the day of Pentecost (Acts 2:1-41And when the day of Pentecost was fully come, they were all with one accord in one place. 2And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. 3And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. 4And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance. (Acts 2:1‑4)). It was a new "beginning" in the ways of God (Acts 11:1515And as I began to speak, the Holy Ghost fell on them, as on us at the beginning. (Acts 11:15)). It was not a revival in God’s dealings with Israel, as was the case in Hezekiah's and Josiah's day, but an entirely new thing in the ways of God. This new thing—the Church or Assembly—was formed by the baptism of the Holy Spirit (Matt. 3:1111I indeed baptize you with water unto repentance: but he that cometh after me is mightier than I, whose shoes I am not worthy to bear: he shall baptize you with the Holy Ghost, and with fire: (Matthew 3:11); Mark 1:88I indeed have baptized you with water: but he shall baptize you with the Holy Ghost. (Mark 1:8); Luke 3:1616John answered, saying unto them all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire: (Luke 3:16); John 1:3333And I knew him not: but he that sent me to baptize with water, the same said unto me, Upon whom thou shalt see the Spirit descending, and remaining on him, the same is he which baptizeth with the Holy Ghost. (John 1:33); Acts 1:5; 11:165For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence. (Acts 1:5)
16Then remembered I the word of the Lord, how that he said, John indeed baptized with water; but ye shall be baptized with the Holy Ghost. (Acts 11:16)
; 1 Cor. 12:1313For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit. (1 Corinthians 12:13)). (See Baptism of the Holy Spirit.) Scripture clearly indicates that the Church was not in existence before that inaugural moment when the Spirit of God came to dwell in the believers gathered in the upper room. It, therefore, could not have existed in Old Testament times, nor could it have existed in the days of the Lord's earthly ministry. The following four points are proofs of this:
•  CHRIST'S MINISTRY—In the days of the Lord's earthly ministry, He taught His disciples that He would build the Church at some future time. He said, "Upon this rock I will build My Church" (Matt. 16:1818And I say also unto thee, That thou art Peter, and upon this rock I will build my church; and the gates of hell shall not prevail against it. (Matthew 16:18)). Clearly, it was not in existence at that time.
•  CHRIST'S DEATHEphesians 2:14-1614For he is our peace, who hath made both one, and hath broken down the middle wall of partition between us; 15Having abolished in his flesh the enmity, even the law of commandments contained in ordinances; for to make in himself of twain one new man, so making peace; 16And that he might reconcile both unto God in one body by the cross, having slain the enmity thereby: (Ephesians 2:14‑16) states that one of the things that characterizes the Church is that "the middle wall of partition" between believing Jews and Gentiles has been done away with, and the "enmity" that existed between them has been slain. This, Paul says, has been done in Christ's death on "the cross." This means that the Church could not have been in existence before Christ died.
•  CHRIST'S SENDING OF THE HOLY SPIRIT1 Corinthians 12:1313For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit. (1 Corinthians 12:13) states that the Church was formed by the coming of the Holy Spirit to dwell in this new company of believers. This did not happen until Pentecost.
The Church can be seen in Scripture in at least twelve different figures, depicting its various aspects. They are:
•  A BODY—the oneness that exists among the members (Rom. 12:4-54For as we have many members in one body, and all members have not the same office: 5So we, being many, are one body in Christ, and every one members one of another. (Romans 12:4‑5); Eph. 4:1-161I therefore, the prisoner of the Lord, beseech you that ye walk worthy of the vocation wherewith ye are called, 2With all lowliness and meekness, with longsuffering, forbearing one another in love; 3Endeavoring to keep the unity of the Spirit in the bond of peace. 4There is one body, and one Spirit, even as ye are called in one hope of your calling; 5One Lord, one faith, one baptism, 6One God and Father of all, who is above all, and through all, and in you all. 7But unto every one of us is given grace according to the measure of the gift of Christ. 8Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men. 9(Now that he ascended, what is it but that he also descended first into the lower parts of the earth? 10He that descended is the same also that ascended up far above all heavens, that he might fill all things.) 11And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers; 12For the perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ: 13Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ: 14That we henceforth be no more children, tossed to and fro, and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive; 15But speaking the truth in love, may grow up into him in all things, which is the head, even Christ: 16From whom the whole body fitly joined together and compacted by that which every joint supplieth, according to the effectual working in the measure of every part, maketh increase of the body unto the edifying of itself in love. (Ephesians 4:1‑16)).
•  A BRIDE—Christ’s love and affection for it (Eph. 5:26-3126That he might sanctify and cleanse it with the washing of water by the word, 27That he might present it to himself a glorious church, not having spot, or wrinkle, or any such thing; but that it should be holy and without blemish. 28So ought men to love their wives as their own bodies. He that loveth his wife loveth himself. 29For no man ever yet hated his own flesh; but nourisheth and cherisheth it, even as the Lord the church: 30For we are members of his body, of his flesh, and of his bones. 31For this cause shall a man leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh. (Ephesians 5:26‑31); Rev. 19:7-9; 21:2, 97Let us be glad and rejoice, and give honor to him: for the marriage of the Lamb is come, and his wife hath made herself ready. 8And to her was granted that she should be arrayed in fine linen, clean and white: for the fine linen is the righteousness of saints. 9And he saith unto me, Write, Blessed are they which are called unto the marriage supper of the Lamb. And he saith unto me, These are the true sayings of God. (Revelation 19:7‑9)
2And I John saw the holy city, new Jerusalem, coming down from God out of heaven, prepared as a bride adorned for her husband. (Revelation 21:2)
9And there came unto me one of the seven angels which had the seven vials full of the seven last plagues, and talked with me, saying, Come hither, I will show thee the bride, the Lamb's wife. (Revelation 21:9)
•  A CANDLESTICK—Local assembly privileges and testimony (Rev. 1:12, 20: 3:22).
•  A CITY—It’s administration in the world to come (Rev. 21:9-22:59And there came unto me one of the seven angels which had the seven vials full of the seven last plagues, and talked with me, saying, Come hither, I will show thee the bride, the Lamb's wife. 10And he carried me away in the spirit to a great and high mountain, and showed me that great city, the holy Jerusalem, descending out of heaven from God, 11Having the glory of God: and her light was like unto a stone most precious, even like a jasper stone, clear as crystal; 12And had a wall great and high, and had twelve gates, and at the gates twelve angels, and names written thereon, which are the names of the twelve tribes of the children of Israel: 13On the east three gates; on the north three gates; on the south three gates; and on the west three gates. 14And the wall of the city had twelve foundations, and in them the names of the twelve apostles of the Lamb. 15And he that talked with me had a golden reed to measure the city, and the gates thereof, and the wall thereof. 16And the city lieth foursquare, and the length is as large as the breadth: and he measured the city with the reed, twelve thousand furlongs. The length and the breadth and the height of it are equal. 17And he measured the wall thereof, an hundred and forty and four cubits, according to the measure of a man, that is, of the angel. 18And the building of the wall of it was of jasper: and the city was pure gold, like unto clear glass. 19And the foundations of the wall of the city were garnished with all manner of precious stones. The first foundation was jasper; the second, sapphire; the third, a chalcedony; the fourth, an emerald; 20The fifth, sardonyx; the sixth, sardius; the seventh, chrysolite; the eighth, beryl; the ninth, a topaz; the tenth, a chrysoprasus; the eleventh, a jacinth; the twelfth, an amethyst. 21And the twelve gates were twelve pearls; every several gate was of one pearl: and the street of the city was pure gold, as it were transparent glass. 22And I saw no temple therein: for the Lord God Almighty and the Lamb are the temple of it. 23And the city had no need of the sun, neither of the moon, to shine in it: for the glory of God did lighten it, and the Lamb is the light thereof. 24And the nations of them which are saved shall walk in the light of it: and the kings of the earth do bring their glory and honor into it. 25And the gates of it shall not be shut at all by day: for there shall be no night there. 26And they shall bring the glory and honor of the nations into it. 27And there shall in no wise enter into it any thing that defileth, neither whatsoever worketh abomination, or maketh a lie: but they which are written in the Lamb's book of life. 1And he showed me a pure river of water of life, clear as crystal, proceeding out of the throne of God and of the Lamb. 2In the midst of the street of it, and on either side of the river, was there the tree of life, which bare twelve manner of fruits, and yielded her fruit every month: and the leaves of the tree were for the healing of the nations. 3And there shall be no more curse: but the throne of God and of the Lamb shall be in it; and his servants shall serve him: 4And they shall see his face; and his name shall be in their foreheads. 5And there shall be no night there; and they need no candle, neither light of the sun; for the Lord God giveth them light: and they shall reign for ever and ever. (Revelation 21:9‑22:5)).