The word means “one sent forth.” It was the title Jesus gave (Luke 6:13) to the Twelve whom He chose and ordained (John 15:16) to be His closest disciples during His ministry on earth and whom He sent forth to represent Him after His Ascension into heaven. The calling of an Apostle is to be a special witness of the name of Jesus Christ in all the world, particularly of His divinity and of His bodily resurrection from the dead (Acts 1:22; D&C 107:23).
Twelve men with this high calling constitute an administrative council in the work of the ministry. When a vacancy occurred with the death of Judas Iscariot, Matthias was divinely appointed to that special office as a member of the council (Acts 1:15–26). Today twelve men with this same divine calling and ordination constitute the Quorum of the Twelve Apostles in The Church of Jesus Christ of Latter-day Saints.
The title was also applied to others who, though not of the number of the original Twelve, yet were called to serve as special witnesses of the Lord. Paul repeatedly spoke of himself as an Apostle (Rom. 1:1; 1 Cor. 1:1; 9:1; Gal. 1:1). He applied the title to James, the Lord’s brother (Gal. 1:19), and also to Barnabas (1 Cor. 9:5–6; see also Acts 14:14). The New Testament does not inform us whether these three brethren also served in the Council of the Twelve as vacancies occurred therein, or whether they were Apostles strictly in the sense of being special witnesses for the Lord Jesus Christ.
Jesus is referred to as an Apostle in Heb. 3:1–2, a designation meaning that He is the personal and select representative of the Father.