The word "apostle" has two meanings, the broader meaning of a messenger and the narrow meaning of an early apostle directly linked to Jesus Christ. The more general meaning of the word is translated into Latin as 'missio', and from this word we get 'missionary.'
The word apostle comes from the Greek word ἀπόστολος (apostolos). According to the Bauer lexicon, Walter Bauer's Greek-English Lexicon of the NT: "…Judaism had an office known as apostle (שליח)". See also Proselytes. 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.