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:

Buckeye \Buck"eye`\ (b[u^]k"[imac]`), n.
   1. (Bot.) A name given to several American trees and shrubs
      of the same genus ([AE]sculus) as the horse chestnut.
      [1913 Webster]

   The Ohio buckeye, or Fetid buckeye, is Aesculus glabra.


   Red buckeye is Aesculus Pavia.

   Small buckeye is Aesculus paviflora.

   Sweet buckeye, or Yellow buckeye, is Aesculus flava.
      [1913 Webster]

   2. A cant name for a native or resident of Ohio. [U.S.]
      [1913 Webster]

   Buckeye State, Ohio; -- so called because buckeye trees
      abound there.
      [1913 Webster]

WordNet (r) 3.0 (2006):

buckeye
    n 1: the inedible nutlike seed of the horse chestnut [syn:
         buckeye, horse chestnut, conker]
    2: tree having palmate leaves and large clusters of white to red
       flowers followed by brown shiny inedible seeds [syn: horse
       chestnut, buckeye, Aesculus hippocastanum]
    3: a native or resident of Ohio [syn: Ohioan, Buckeye]