James E. Talmage
"Discipleship is general; any follower of a man or devotee to a principle may be called a disciple, The Holy Apostleship is an office and calling belonging to the Higher or Melchizedek Priesthood, at once exalted and specific, comprising as a distinguishing function that of personal and special witness to the divinity of Jesus Christ as the one and only Redeemer and Savior of mankind. The apostleship is an individual bestowal, and as such is conferred only through ordination. That the Twelve did constitute a council or 'quorum' having authority in the Church established by Jesus Christ is shown by their ministrations after the Lord's resurrection and ascension.
"….The word 'apostle' is an Anglicized form derived from the Greek apostolos, meaning literally 'one who is sent,' and connoting an envoy or official messenger, who speaks and acts by the authority of one superior to himself." (Jesus the Christ, p. 227-8)