The Collaborative International Dictionary of English v.0.48:

Horseshoe \Horse"shoe`\, n.
   1. A shoe for horses, consisting of a narrow plate of iron in
      form somewhat like the letter U, nailed to a horse's hoof.
      [1913 Webster]

   2. Anything shaped like a horsehoe, such as a U-shaped bend
      in a river.
      [1913 Webster]

   3. (Zool.) The Limulus or horsehoe crab.
      [1913 Webster]

   4. pl. A game in which horseshoes or horseshoe-shaped objects
      (usually made of metal) are thrown at either of two stakes
      fixed in the ground at a distance of 30 to 40 feet apart.
      The player stands at or near one stake and throws several
      the horseshoes at the other stake. Points are scored when
      the player throws the horseshoe so that it surrounds the
      stake; fewer points are scored if the horseshoe is close
      to but not surrounding the stake. The players take turns
      and the first player to achieve the target score wins; as,
      "almost" only counts with hand grenades or in horseshoes.
      [PJC]

   Horsehoe head (Med.), an old name for the condition of the
      skull in children, in which the sutures are too open, the
      coronal suture presenting the form of a horsehoe.
      --Dunglison.

   Horsehoe magnet, an artificial magnet in the form of a
      horsehoe.

   Horsehoe nail. See Horsenail.

   Horsehoe nose (Zool.), a bat of the genus Rhinolophus,
      having a nasal fold of skin shaped like a horsehoe.
      [1913 Webster]

WordNet (r) 3.0 (2006):

horseshoe
    n 1: game equipment consisting of an open ring of iron used in
         playing horseshoes
    2: U-shaped plate nailed to underside of horse's hoof [syn:
       horseshoe, shoe]
    v 1: equip (a horse) with a horseshoe or horseshoes