The Collaborative International Dictionary of English v.0.48:

Horse-chestnut \Horse`-chest"nut\, Horsechestnut
\Horse`chest"nut\, n.
   1. (Bot.) The large nutlike seed of a species of Aesculus
      (Aesculus Hippocastanum), formerly ground, and fed to
      horses, whence the name. The seed is not considered edible
      by humans. [WordNet sense 2]
      [1913 Webster + WordNet 1.5]

   2. (Bot.) The tree itself (Aesculus hippocastanum), which
      was brought from Constantinople in the beginning of the
      sixteenth century, and is now common in the temperate
      zones of both hemispheres; it has palmate leaves and large
      clusters of white to red flowers followed by brown shiny
      inedible seeds. The native American species is also called
      buckeye and conker. [WordNet sense 1]
      [1913 Webster + WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

conker \conk"er\ (k[o^][ng]k"[~e]r), n.
   the inedible nutlike seed of the horsechestnut.

   Syn: buckeye, horsechestnut, horse-chestnut.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

conker
    n 1: the inedible nutlike seed of the horse chestnut [syn:
         buckeye, horse chestnut, conker]