The Collaborative International Dictionary of English v.0.48:

Carrier \Car"ri*er\, n. [From Carry.]
   1. One who, or that which, carries or conveys; a messenger.
      [1913 Webster]

            The air which is but . . . a carrier of the sounds.
                                                  --Bacon.
      [1913 Webster]

   2. One who is employed, or makes it his business, to carry
      goods for others for hire; a porter; a teamster.
      [1913 Webster]

            The roads are crowded with carriers, laden with rich
            manufactures.                         --Swift.
      [1913 Webster]

   3. (Mach.) That which drives or carries; as:
      (a) A piece which communicates to an object in a lathe the
          motion of the face plate; a lathe dog.
      (b) A spool holder or bobbin holder in a braiding machine.
          (c) A movable piece in magazine guns which transfers
          the cartridge to a position from which it can be
          thrust into the barrel.
          [1913 Webster]

   Carrier pigeon (Zool.), a variety of the domestic pigeon
      used to convey letters from a distant point to to its
      home.

   Carrier shell (Zool.), a univalve shell of the genus
      Phorus; -- so called because it fastens bits of stones
      and broken shells to its own shell, to such an extent as
      almost to conceal it.

   Common carrier (Law.) See under Common, a.
      [1913 Webster]

WordNet (r) 3.0 (2006):

carrier
    n 1: someone whose employment involves carrying something; "the
         bonds were transmitted by carrier" [syn: carrier,
         bearer, toter]
    2: a self-propelled wheeled vehicle designed specifically to
       carry something; "refrigerated carriers have revolutionized
       the grocery business"
    3: a large warship that carries planes and has a long flat deck
       for takeoffs and landings [syn: aircraft carrier,
       carrier, flattop, attack aircraft carrier]
    4: an inactive substance that is a vehicle for a radioactive
       tracer of the same substance and that assists in its recovery
       after some chemical reaction
    5: a person or firm in the business of transporting people or
       goods or messages [syn: carrier, common carrier]
    6: a radio wave that can be modulated in order to transmit a
       signal [syn: carrier wave, carrier]
    7: a man who delivers the mail [syn: mailman, postman, mail
       carrier, letter carrier, carrier]
    8: a boy who delivers newspapers [syn: carrier, newsboy]
    9: (medicine) a person (or animal) who has some pathogen to
       which he is immune but who can pass it on to others [syn:
       carrier, immune carrier]
    10: a rack attached to a vehicle; for carrying luggage or skis
        or the like
    11: (genetics) an organism that possesses a recessive gene whose
        effect is masked by a dominant allele; the associated trait
        is not apparent but can be passed on to offspring