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).
In 1 Corinthians 4:6, 9-13 Paul names Apollos as an apostle who had shared in the privations of the traveling apostolic life (cf. 1 Cor., 9:5; 15:7). Apollos had only been acquainted with the baptism of John but arrived in Ephesus in the early 50s (this is dated from New Testament and external evidence – Acts 18:2 Claudius’ expulsion of Jews from Rome and Acts 18:12 Paul’s appearance before Gallio), and preached in the synagogue ‘accurately’ about Jesus (Acts 18:25). One fascinating fact about Apollos is that Priscilla and Aquila expounded some very exact knowledge to him from a tradition called ‘The Way of God’ (Acts 18:26). They had heard something that disturbed them in the synagogue and took him aside, and expounded to him most likely some exact knowledge about baptism into Jesus’ name, and naming Jesus as Messiah. The Greek word for expound is a derivative of ektithmi which means a careful, meticulous outline of the subject matter. The word is used of Paul expounding ‘about the kingdom of God, and from the Law of Moses and from the Prophets’ from early in the morning until late at night in his house in Rome – converting some but not others. It is also used in a similar way of Peter in Acts 11:4, and by Josephus (Ant. 12.1.2) to describe a ‘particular account, with great exactness’ that he will soon give about the Arabians. If anyone is concerned that Paul said ‘a woman’ could not teach (didaskalos) a man needs to weigh up this evidence from Acts 18:26 that uses a far more powerful verb to describe a woman expounding a very exact Christian understanding to an apostle. Also, ‘the way of the Lord’ (v.25) that Apollos was instructed in and ‘The Way of God’ (v.26) that Priscilla and Aquila instructed Apollos from were likely two of the earliest named oral and/or textual traditions about John the Baptist and Jesus. So Priscilla and Aquila and Apollos were also ‘keepers’ of two of the earliest known traditions about John and Jesus. They were all also co-workers of Paul.