While Jesus named only 12 apostles during his ministry, the title of "apostle" is often applied to other New Testament figures.
St. Paul is among the most widely known apostle, even though he was never associated with the Twelve until after Jesus' death and resurrection.
In fact, St. Paul even persecuted the apostles prior to his conversion.
St. Barnabas the apostle
St. Barnabas is another example of a New Testament figure who was given the title of "apostle."
What's interesting is that the author of the Acts of the Apostles gives both Paul and Barnabas this title, "the apostles Barnabas and Paul" (Acts 14:14).
The English word “apostle” is derived from the Greek apostello, “to send forth.” An apostle is one who is sent by God to preach the Gospel to the nations.
As a result, while Paul and Barnabas were not originally chosen by Jesus to be part of the Twelve, the Church believed that they shared in the same ministry, having been called by God to preach the Gospel.
With this definition of the word, "apostle," there have been many men and women throughout the centuries who have been chosen and sent by God. This is why there are saints who were called the "Apostle of Ireland," or "Apostle of Germany," or even, "Apostle of Rome."
The Catholic Encyclopedia confirms the variety of meanings for this one word:
It is at once evident that in a Christian sense, everyone who had received a mission from God, or Christ, to man could be called "Apostle". In fact, however, it was reserved to those of the disciples who received this title from Christ. At the same time, like other honorable titles, it was occasionally applied to those who in some way realized the fundamental idea of the name.
In the end, Ii all depends on how you define the word as to know who you are referring to.
