“Someone who is sent,” often “a messenger.” In the New Testament the word refers particularly to twelve men whom Jesus selected to be with him and whom he sent out to preach and to cast out demons (Mark 3:14-15). Other individuals than the Twelve bore that title--for example, Paul and Barnabas (Acts 14:14). Apostles were important figures in the early church (1 Cor. 12:28). They were appointed by Christ, not by men (Gal. 1:1), and they gave authoritative witness to what God had done in Christ (Acts 1:22).

The Shaw Pocket Bible Handbook, Walter A. Elwell, Editor, (Harold Shaw Publ., Wheaton , IL; 1984), p. 346