Concise Bible Dictionary:
1. From γεμμάω, “to beget.” This word is used
a) for the natural generation of mankind, as in Matthew 1.
c) for the Lord Jesus, when He became a man, conceived (begotten) of the Holy Ghost (Matt. 1:2020But while he thought on these things, behold, the angel of the Lord appeared unto him in a dream, saying, Joseph, thou son of David, fear not to take unto thee Mary thy wife: for that which is conceived in her is of the Holy Ghost. (Matthew 1:20)); and declared in those words of Jehovah to Him, “Thou art my Son; this day have I begotten thee” (Psa. 2:77I will declare the decree: the Lord hath said unto me, Thou art my Son; this day have I begotten thee. (Psalm 2:7); Acts 13:3333God hath fulfilled the same unto us their children, in that he hath raised up Jesus again; as it is also written in the second psalm, Thou art my Son, this day have I begotten thee. (Acts 13:33); Heb. 1:55For unto which of the angels said he at any time, Thou art my Son, this day have I begotten thee? And again, I will be to him a Father, and he shall be to me a Son? (Hebrews 1:5); Heb. 5:55So also Christ glorified not himself to be made an high priest; but he that said unto him, Thou art my Son, to day have I begotten thee. (Hebrews 5:5)).
d) μόνοτγενής, a name of peculiar affection for the Lord Jesus in His eternal Sonship as existing before He came into the world, and referred to as the only begotten Son, the only begotten of the Father (John 1:14, 1814And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth. (John 1:14)
18No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him. (John 1:18); John 3:16,1816For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. (John 3:16)
18He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God. (John 3:18)). It is here used as the equivalent of a Hebrew word (yachid) which signifies “only one,” and hence “darling” (Gen. 22:22And he said, Take now thy son, thine only son Isaac, whom thou lovest, and get thee into the land of Moriah; and offer him there for a burnt offering upon one of the mountains which I will tell thee of. (Genesis 22:2); Psa. 22:2020Deliver my soul from the sword; my darling from the power of the dog. (Psalm 22:20); Psa. 35:1717Lord, how long wilt thou look on? rescue my soul from their destructions, my darling from the lions. (Psalm 35:17)).
2. πρωτότοκος, applied to the Lord Jesus as “the first-begotten,” or rather “the firstborn” as marking His supremacy above all. The angels were called to worship Him when He was brought into the world (Heb. 1:66And again, when he bringeth in the firstbegotten into the world, he saith, And let all the angels of God worship him. (Hebrews 1:6)); and He is said to be the firstborn from among the dead (Rev. 1:55And from Jesus Christ, who is the faithful witness, and the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and washed us from our sins in his own blood, (Revelation 1:5)).