The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A two-horned animal of some unknown kind, so called in the
      Authorized Version of the Scriptures.
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            Canst thou bind the unicorn with his band in the
            furrow?                               --Job xxxix.
                                                  10.
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   Note: The unicorn mentioned in the Scripture was probably the
         urus. See the Note under Reem.
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   3. (Zool.)
      (a) Any large beetle having a hornlike prominence on the
          head or prothorax.
      (b) The larva of a unicorn moth.
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   4. (Zool.) The kamichi; -- called also unicorn bird.
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   5. (Mil.) A howitzer. [Obs.]
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   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 (Zool.), the narwhal.

   Unicorn moth (Zool.), a notodontian moth (Coelodasys
      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 (Chamaelirium luteum). Both are used in
      medicine.

   Unicorn shell (Zool.), 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.
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The Collaborative International Dictionary of English v.0.48:

Colicroot \Col"ic*root`\, n.
   A bitter American herb of the Bloodwort family, with the
   leaves all radical, and the small yellow or white flowers in
   a long spike (Aletris farinosa and Aletris aurea). Called
   sometimes star grass, blackroot, blazing star, and
   unicorn root.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unicorn root
    n 1: any of several perennials of the genus Aletris having
         grasslike leaves and bitter roots reputed to cure colic
         [syn: colicroot, colic root, crow corn, star grass,
         unicorn root]