His name in Aramaic (Teʾoma) and Greek (Didymos) means “twin”; John 11:16 identifies him as “Thomas, called the Twin.” He is called Judas Thomas (i.e., Judas the Twin) by the Syrians. Thomas's character is outlined in The Gospel According to John.

Why was Thomas called Didymus?

In both the Book of John, one of the Gospels of the New Testament, and in the apocryphal Acts of Thomas, Thomas is described as “Thomas, who is called Didymus,” a redundancy, since “Thomas” comes from the Aramaic word teoma, meaning “twin” (in Hebrew, it's te'om), for which the word in Greek is didymus.

Which apostle is also called Didymus?

Didymus comes from the ancient Greek word for twin, while Thomas comes from the Aramaic word, also meaning twin. This would suggest that the Apostle Thomas' real name was actually Judas – not THAT Judas – and was referred to as 'Twin Judas Twin' and was one of Christ's brothers.