The word apostle (Greek: apostolos) derives from the verb “to send” (Greek: apostello). It carries the sense of someone being sent for a purpose, and is probably used in the New Testament to refer to those sent by God for a specific purpose.
Those called “apostle” includes the Twelve (Mark 6:30), James the brother of Jesus (Gal 1:19), Barnabas (Acts 14:4) and probably Andronicus and Junia (Rom 16:7). Paul designates himself as an apostle (1 Cor 15:9).
In summary, apostles are those sent by God for a specific purpose; this includes Paul who received his mission directly from the risen Lord Jesus (Acts 9:15).