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

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.”

St. Paul was clearly 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 (1 Cor. 9:1–2).