From the Greek apostolos, meaning one who is sent and enjoys the authority of the agent who instructs him. There was already a Jewish functionary, called a shaliach, who was trusted with the authority of his employer when taking messages. Thus, in the NT the apostles bear witness to the message of Jesus and continue his work.
The twelve disciples in the gospels are called ‘apostles’ by Luke (6: 13; 22: 14; Acts 1: 2) and apparently by Paul (1 Cor. 15: 7) and only once each by Matt. 10: 2 and Mark 6.30); though the names vary slightly, the list is always of twelve, signifying that they are the leaders of an elect race, corresponding to the twelve tribes of Israel. The betrayal of Jesus by Judas created a vacancy in the number, and Matthias was elected to fill it. Subsequent deaths did not create vacancies, for in Luke's view apostles are unique. Yet the office was soon expanded to embrace Paul (unless it is Luke who was responsible for restricting a broader title to the twelve disciples), who claims the title on the basis of his vision of the Risen Christ (1 Cor. 15: 8) and his commissioning to go to the Gentiles. A kind of second rank of apostles apparently included Barnabas (Acts 14: 14) and Junia (Rom. 16: 7—the latter name is feminine) and cf. 2 Pet. 3: 2. The names of the Twelve are: Simon Peter, James and John, Andrew, Philip, Bartholomew (Nathanael?), Matthew (Levi?), Thomas, James the Less, Thaddaeus (Judas the son of James?), Simon the Cananean or Zealot; and Judas Iscariot—afterwards succeeded by Matthias.