Didymus is the second name given for the Apostle Thomas in three places in the Gospel of John: John 11:16; 20:24; 21:2. His two names are actually one and the same. Thomas comes from the Hebrew (or Aramaic) and means "the twin." Didymus is from the Greek and means "the twin." It is likely that Thomas was born as a twin. This would best explain such a name.
Some Bible students have connected his name with his nature. Thomas was known as Doubting Thomas. Perhaps too often he halted between two opinions.