Thaddaeus was one of the 12 original apostles according to the lists in Mark 3:18 and Matthew 10:3. The King James Version identifies him as "Lebbaeus, whose surname was Thaddaeus." It is quite likely that Thaddaeus is the same person as Judas son of James (not Iscariot) in Luke 6:16 and Acts 1:13.
Thaddaeus, the Apostle
From Tyndale Bible Dictionary, adapted by Mission Mutual. CC BY-SA 4.0.