Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
Twin, one of the twelve (Mt. 10:3; Mk. 3:18, etc.). He was also called Didymus (Jn. 11:16, 20:24), which is the Greek equivalent of the Hebrew name. All we know regarding him is recorded in the fourth Gospel (Jn. 11:15, 16, 14:4, 5, 20:24, 25,26-29). From the circumstance that in the lists of the apostles he is always mentioned along with Matthew, who was the son of Alphaeus (Mk. 3:18), and that these two are always followed by James, who was also the son of Alphaeus, it has been supposed that these three, Matthew, Thomas, and James, were brothers.