The word apostolos means “sent one”. There are two categories of apostolos, ones sent by Christ Jesus and ones sent by His ecclesia. The former are the ones Who’s doctrine we are to remain steadfast in and the latter are essentially missionaries who proclaim the doctrine of the former.
There is a definite distinction between the Twelve Apostles of The Lamb who established doctrine and gave us the Gospel once delivered for all the saints, and apostles that are sent out by the church as missionaries. Those sent out by the church as apostles or missionaries don’t create doctrine, they teach The Apostles’ Doctrine.
I've just been catching up on the forums recently, and this pretty fascinating, and, at least from a superficial study, seems to be true.
Throughout the Gospels and most of Acts, the term usually clearly refers specifically to the twelve, (later, minus Judas, and possibly plus Matthias).
Luke 6: 13. When it was day, he called his disciples, and from them he chose twelve, whom he also named apostles:
Acts 1: 26. They drew lots for them, and the lot fell on Matthias, and he was numbered with the eleven apostles.
Initially, Barnabas is is obviously not an Apostle, though he is close to them:
Acts 4: 36-37. Joses, who by the apostles was surnamed Barnabas (which is, being interpreted, Son of Encouragement), a Levite, a man of Cyprus by race, having a field, sold it, and brought the money and laid it at the apostles' feet.
Acts 9:26-27. When Saul had come to Jerusalem, he tried to join himself to the disciples; but they were all afraid of him, not believing that he was a disciple. But Barnabas took him, and brought him to the apostles, and declared to them how he had seen the Lord in the way, and that he had spoken to him, and how at Damascus he had preached boldly in the name of Jesus.
But later, Barnabas and Saul are suddenly called apostles (sent ones).
Acts 14: 11, 14. When the multitude saw what Paul had done, they lifted up their voice, saying in the language of Lycaonia, "The gods have come down to us in the likeness of men!" ... But when the apostles, Barnabas and Paul, heard of it, they tore their clothes, and sprang into the multitude, crying out
So what happened between chapters 9 and 14 that warrants Paul and Barnabas being called "sent ones"? They get sent, of course:
Acts 13: 1-4. Now in the assembly that was at Antioch there were some prophets and teachers: Barnabas, Simeon who was called Niger, Lucius of Cyrene, Manaen the foster brother of Herod the tetrarch, and Saul. As they served the Lord and fasted, the Holy Spirit said, "Separate Barnabas and Saul for me, for the work to which I have called them." Then, when they had fasted and prayed and laid their hands on them, they sent them away. So, being sent out by the Holy Spirit, they went down to Seleucia. From there they sailed to Cyprus.
So after being sent out by the assembly (under the guidance of the Holy Spirit), Paul and Barnabas are referred to as "sent ones". But then in the next chapter, Luke resumes using the term to refer to the original apostles still in Jerusalem:
Acts 15: 2. Therefore when Paul and Barnabas had no small discord and discussion with them, they appointed Paul and Barnabas, and some others of them, to go up to Jerusalem to the apostles and elders about this question.
So Luke appears to use the term apostle in two different ways, depending on if the individual was sent by Christ, or by the assembly (on behalf of the Holy Spirit). This is similar to how "Angel" can mean a spiritual being, or simply a human messenger, depending on context.
Paul seems to make this same distinction in his writing. He frequently refers to himself (and the original apostles in Jerusalem) as an apostle "of Christ":
2 Corinthians 1: 1. Paul, an apostle of Christ Jesus through the will of God, and Timothy our brother, to the assembly of God which is at Corinth, with all the saints who are in the whole of Achaia:
While refering to brothers who have been sent from other assemblies as apostles of those assemblies:
2 Corinthians 8: 23-24. As for Titus, he is my partner and fellow worker for you. As for our brothers, they are the apostles of the assemblies, the glory of Christ. Therefore show the proof of your love to them in front of the assemblies, and of our boasting on your behalf.
In this case rather than planting new churches (as Paul and Barnabas did), these apostles seem to be merely observing the Corinthians on behalf of the other assemblies. Paul arguably makes this same distinction in Galatians
Galatians 1: 1. Paul, an apostle (not from men, neither through man, but through Jesus Christ, and God the Father, who raised him from the dead)
Paul also refers to an apostle (sent one) from the assembly at Ephesus:
Philippians 2: 25-26. But I counted it necessary to send to you Epaphroditus, my brother, fellow worker, fellow soldier, and your apostle and servant of my need; since he longed for you all, and was very troubled, because you had heard that he was sick.
So we see
The Apostles who are the 12(ish) sent
by Christ, and we see generically
apostles, who are sent
by assemblies (and by the Holy Spirit) either to plant new churches, or visit existing ones.
I'll propose there is actually a third class of apostle:
Hebrews 3: 1. Therefore, holy brothers, partakers of a heavenly calling, consider the Apostle and High Priest of our confession, Jesus;
Jesus, of course, was not sent by Himself, nor by an assembly, but rather by His Father.
You might say that these three types of apostle correspond to which member of the Trinity is doing the sending.