Source : Webster's Revised Unabridged Dictionary (1913)

Unicorn \U"ni*corn\, n. [OE. unicorne, F. unicorne, L. unicornis
   one-horned, having a single horn; unus one + cornu a horn;
   cf. L. unicornuus a unicorn. See {One}, and {Horn}.]
   1. A fabulous animal with one horn; the monoceros; -- often
      represented in heraldry as a supporter.

   2. A two-horned animal of some unknown kind, so called in the
      Authorized Version of the Scriptures.

            Canst thou bind the unicorn with his band in the
            furrow?                               --Job xxxix.
                                                  10.

   Note: The unicorn mentioned in the Scripture was probably the
         urus. See the Note under {Reem}.

   3. (Zo["o]l.)
      (a) Any large beetle having a hornlike prominence on the
          head or prothorax.
      (b) The larva of a unicorn moth.

   4. (Zo["o]l.) The kamichi; -- called also {unicorn bird}.

   5. (Mil.) A howitzer. [Obs.]

   {Fossil unicorn}, or {Fossil unicorn's horn} (Med.), a
      substance formerly of great repute in medicine; -- named
      from having been supposed to be the bone or the horn of
      the unicorn.

   {Unicorn fish}, {Unicorn whale} (Zo["o]l.), the narwhal.

   {Unicorn moth} (Zo["o]l.), a notodontian moth ({C[oe]lodasys
      unicornis}) whose caterpillar has a prominent horn on its
      back; -- called also {unicorn prominent}.

   {Unicorn root} (Bot.), a name of two North American plants,
      the yellow-flowered colicroot ({Aletris farinosa}) and the
      blazing star ({Cham[ae]lirium luteum}). Both are used in
      medicine.

   {Unicorn shell} (Zo["o]l.), any one of several species of
      marine gastropods having a prominent spine on the lip of
      the shell. Most of them belong to the genera {Monoceros}
      and {Leucozonia}.

Source : WordNet®

unicorn
     n : an imaginary creature represented as a white horse with a
         long horn growing from its forehead