Boyd’s Bible Dictionary:

(brotherly love). A city of Lydia in Asia Minor, and seat of one of the seven churches of Asia (Rev. 1:11; 3:7-1311Saying, I am Alpha and Omega, the first and the last: and, What thou seest, write in a book, and send it unto the seven churches which are in Asia; unto Ephesus, and unto Smyrna, and unto Pergamos, and unto Thyatira, and unto Sardis, and unto Philadelphia, and unto Laodicea. (Revelation 1:11)
7And to the angel of the church in Philadelphia write; These things saith he that is holy, he that is true, he that hath the key of David, he that openeth, and no man shutteth; and shutteth, and no man openeth; 8I know thy works: behold, I have set before thee an open door, and no man can shut it: for thou hast a little strength, and hast kept my word, and hast not denied my name. 9Behold, I will make them of the synagogue of Satan, which say they are Jews, and are not, but do lie; behold, I will make them to come and worship before thy feet, and to know that I have loved thee. 10Because thou hast kept the word of my patience, I also will keep thee from the hour of temptation, which shall come upon all the world, to try them that dwell upon the earth. 11Behold, I come quickly: hold that fast which thou hast, that no man take thy crown. 12Him that overcometh will I make a pillar in the temple of my God, and he shall go no more out: and I will write upon him the name of my God, and the name of the city of my God, which is new Jerusalem, which cometh down out of heaven from my God: and I will write upon him my new name. 13He that hath an ear, let him hear what the Spirit saith unto the churches. (Revelation 3:7‑13)

Concise Bible Dictionary:

The assembly in this city was one of the “seven churches in Asia” to which the addresses in the Revelation were sent. The address to Philadelphia shows that the church there was characterized by little strength, but by faithfulness. If the seven addresses be viewed historically, this one comes after those representing Popery and Protestantism, intimating that when all hope of restoring the church is over, there may still be found a company keeping Christ’s word and not denying His name. See REVELATION.

Strong’s Dictionary of Greek Words:

from Φιλάδελφος (the same as 5361), a king of Pergamos; Philadelphia, a place in Asia Minor
KJV Usage:

Jackson’s Dictionary of Scripture Proper Names:

brotherly love

Potts’ Bible Proper Names:

a place in Asia Minor, Rev. 1:11. {Amor fratris-Love of a brother; brotherly love:}