The Collaborative International Dictionary of English v.0.48:

Apostle \A*pos"tle\, n. [OE. apostle, apostel, postle, AS.
   apostol, L. apostolus, fr. Gr. ? messenger, one sent forth or
   away, fr. ? to send off or away; ? from + ? to send; akin to
   G. stellen to set, E. stall: cf. F. ap[^o]tre, Of. apostre,
   apostle, apostele, apostole.]
   1. Literally: One sent forth; a messenger. Specifically: One
      of the twelve disciples of Christ, specially chosen as his
      companions and witnesses, and sent forth to preach the
      gospel.
      [1913 Webster]

            He called unto him his disciples, and of them he
            chose twelve, whom also he named apostles. --Luke
                                                  vi. 13.
      [1913 Webster]

   Note: The title of apostle is also applied to others, who,
         though not of the number of the Twelve, yet were equal
         with them in office and dignity; as, "Paul, called to
         be an apostle of Jesus Christ." --1 Cor. i. 1. In
         --Heb. iii. 1, the name is given to Christ himself, as
         having been sent from heaven to publish the gospel. In
         the primitive church, other ministers were called
         apostles --(Rom. xvi. 7).
         [1913 Webster]

   2. The missionary who first plants the Christian faith in any
      part of the world; also, one who initiates any great moral
      reform, or first advocates any important belief; one who
      has extraordinary success as a missionary or reformer; as,
      Dionysius of Corinth is called the apostle of France, John
      Eliot the apostle to the Indians, Theobald Mathew the
      apostle of temperance.
      [1913 Webster]

   3. (Civ. & Admiralty Law) A brief letter dimissory sent by a
      court appealed from to the superior court, stating the
      case, etc.; a paper sent up on appeals in the admiralty
      courts. --Wharton. Burrill.
      [1913 Webster]

   Apostles' creed, a creed of unknown origin, which was
      formerly ascribed to the apostles. It certainly dates back
      to the beginning of the sixth century, and some assert
      that it can be found in the writings of Ambrose in the
      fourth century.

   Apostle spoon (Antiq.), a spoon of silver, with the handle
      terminating in the figure of an apostle. One or more were
      offered by sponsors at baptism as a present to the
      godchild. --B. Jonson.
      [1913 Webster]

WordNet (r) 3.0 (2006):

apostle
    n 1: an ardent early supporter of a cause or reform; "an apostle
         of revolution"
    2: any important early teacher of Christianity or a Christian
       missionary to a people [syn: Apostle, Apostelic Father]
    3: (New Testament) one of the original 12 disciples chosen by
       Christ to preach his gospel

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "apostle":
   Aaronic priesthood, Ambrose of Milan, Athanasius, Barnabas, Basil,
   Clement of Alexandria, Clement of Rome, Cyprian of Carthage,
   Cyril of Jerusalem, Gregory of Nyssa, Hermas, Ignatius, Irenaeus,
   Jerome, John, John Chrysostom, Justin Martyr, Lactantius Firmianus,
   Luke, Mark, Melchizedek priesthood, Origen, Papias, Paul, Peter,
   Polycarp, Seventy, Tertullian, ante-Nicene Fathers, bishop,
   colporteur, convert, converter, deacon, disciple, elder,
   evangelist, follower, high priest, missionary, missioner,
   patriarch, priest, propagandist, proselyte, proselyter,
   proselytizer, saint, teacher



Easton's 1897 Bible Dictionary:

Apostle
   a person sent by another; a messenger; envoy. This word is once
   used as a descriptive designation of Jesus Christ, the Sent of
   the Father (Heb. 3:1; John 20:21). It is, however, generally
   used as designating the body of disciples to whom he intrusted
   the organization of his church and the dissemination of his
   gospel, "the twelve," as they are called (Matt. 10:1-5; Mark
   3:14; 6:7; Luke 6:13; 9:1). We have four lists of the apostles,
   one by each of the synoptic evangelists (Matt. 10:2-4; Mark
   3:16; Luke 6:14), and one in the Acts (1:13). No two of these
   lists, however, perfectly coincide.

     Our Lord gave them the "keys of the kingdom," and by the gift
   of his Spirit fitted them to be the founders and governors of
   his church (John 14:16, 17, 26; 15:26, 27; 16:7-15). To them, as
   representing his church, he gave the commission to "preach the
   gospel to every creature" (Matt. 28:18-20). After his ascension
   he communicated to them, according to his promise, supernatural
   gifts to qualify them for the discharge of their duties (Acts
   2:4; 1 Cor. 2:16; 2:7, 10, 13; 2 Cor. 5:20; 1 Cor. 11:2). Judas
   Iscariot, one of "the twelve," fell by transgression, and
   Matthias was substituted in his place (Acts 1:21). Saul of
   Tarsus was afterwards added to their number (Acts 9:3-20; 20:4;
   26:15-18; 1 Tim. 1:12; 2:7; 2 Tim. 1:11).

     Luke has given some account of Peter, John, and the two
   Jameses (Acts 12:2, 17; 15:13; 21:18), but beyond this we know
   nothing from authentic history of the rest of the original
   twelve. After the martyrdom of James the Greater (Acts 12:2),
   James the Less usually resided at Jerusalem, while Paul, "the
   apostle of the uncircumcision," usually travelled as a
   missionary among the Gentiles (Gal. 2:8). It was characteristic
   of the apostles and necessary (1) that they should have seen the
   Lord, and been able to testify of him and of his resurrection
   from personal knowledge (John 15:27; Acts 1:21, 22; 1 Cor. 9:1;
   Acts 22:14, 15). (2.) They must have been immediately called to
   that office by Christ (Luke 6:13; Gal. 1:1). (3.) It was
   essential that they should be infallibly inspired, and thus
   secured against all error and mistake in their public teaching,
   whether by word or by writing (John 14:26; 16:13; 1 Thess.
   2:13).

     (4.) Another qualification was the power of working miracles
   (Mark 16:20; Acts 2:43; 1 Cor. 12:8-11). The apostles therefore
   could have had no successors. They are the only authoritative
   teachers of the Christian doctrines. The office of an apostle
   ceased with its first holders.

     In 2 Cor. 8:23 and Phil. 2:25 the word "messenger" is the
   rendering of the same Greek word, elsewhere rendered "apostle."