The term ἀπόστελος seems to have an interesting range of meanings.  As best I can tell, it can be broken up into three senses:

1. The classical sense of “expedition” or ambassador.
2. The koine sense of messenger or ambassador.
3. The Christian sense of the spiritual office of an Apostle.

This first sense does not seem to appear in the New Testament at all, but many teachers make a great deal about this origin for the word.  Perhaps they are right to do so because there is almost no usage of this word in previous Jewish-Greek writings (apparently one reference in Jos.)

This second sense, however, is used often enough for the cautious student of the Bible to take care in his exegesis of particular passages.  Perhaps the best example of this usage is Phil 2:25 where it is clear that Epaphroditus is not being referred to as one of Christ’s Apostles.

Far more common than I expected however, is the technical sense of the office of Apostle.  This term seems to have it’s origin in the event that is recorded in Mt 10:1ff, Mk 3:13ff and Luk 6:12ff.  Interestingly, in all the Gospels except for Luke these are the only instances of the term.  However, in Luke and Acts the term is employed with fair regularity to refer to the twelve.  In all the accounts of this event there is a connection with the selection of the Apostles and the ideas of authority and evangelism.  In the Mathew account, after the roll call, there is a connection with the verb form of the word.

Paul also uses this word in several discussions of his authority and the authority of those in Jerusalem.  Others besides the twelve are also called apostles, such as Barnabas and Silas.  Also of note is the discussion on false-apostles in 2 Cor 11:12-15.