Terminology
The word "apostle" comes from the Greek word ἀπόστολος (apóstólos). By breaking it down into its constituents it gives us a literal translation of "from the fleet". By deriving ἀπό (apó) meaning from and στολος (stólos) meaning fleet.
The more general meaning of the word as one who is sent on a mission is translated into Latin as missio, and from this word we get missionary.
According to the Greek-English Lexicon of the New Testament, "...Judaism had an office known as apostle (שליח)". The Friberg Greek Lexicon gives a broad definition as one who is sent on a mission, a commissioned representative of a congregation, a messenger for God, a person who has the special task of founding and establishing churches. The UBS Greek Dictionary also describes an apostle broadly as a messenger. The Louw-Nida Lexicon gives a very narrow definition of a special messenger, generally restricted to the immediate followers of Jesus, or extended to some others like Paul or other early Christians active in proclaiming the gospel.
Read more about this topic: Twelve Apostles