The Collaborative International Dictionary of English v.0.48:

Buckwheat \Buck"wheat`\, n. [Buck a beech tree + wheat; akin to
   D. boekweit, G. buchweizen.]
   1. (Bot.) A plant (Fagopyrum esculentum) of the Polygonum
      family, the seed of which is used for food.
      [1913 Webster]

   2. The triangular seed used, when ground, for griddle cakes,
      etc.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fagopyrum \Fagopyrum\ prop. n.
   a genus of plants of the buckwheat family, including the
   buckwheat Fagopyrum esculentum; in some classifications
   included in the genus Polygonum.

   Syn: genus Fagopyrum.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

Fagopyrum esculentum
    n 1: a member of the genus Fagopyrum; annual Asian plant with
         clusters of small pinkish white flowers and small edible
         triangular seeds which are used whole or ground into flour
         [syn: buckwheat, Polygonum fagopyrum, Fagopyrum
         esculentum]