Was Paul an apostle?

[spf]

Why is Paul referred to as an apostle when he clearly was not one of the Twelve?

 

[Jim_Blackburn]

The Catholic Encyclopedia defines “apostle” as “one who is sent forth, dispatched–in other words, who is entrusted with a mission, rather, a foreign mission. It has, however, a stronger sense than the word messenger, and means as much as a delegate.” See:

• Apostles
  St. Paul clearly was sent forth by Jesus, entrusted with a mission. In our Lord’s own words, Paul became “a chosen instrument of mine to carry my name before the Gentiles and kings and the sons of Israel” (Acts 9:15).

Paul refers to himself as an apostle many times in the New Testament and he even defends his apostleship in his first epistle to the Corinthians (cf. 1Cor 9:1-2).