Source : Webster's Revised Unabridged Dictionary (1913)

Buck bean \Buck" bean`\ (b[=e]n`). (Bot.)
   A plant ({Menyanthes trifoliata}) which grows in moist and
   boggy places, having racemes of white or reddish flowers and
   intensely bitter leaves, sometimes used in medicine; marsh
   trefoil; -- called also {bog bean}.

Source : WordNet®

Menyanthes trifoliata
     n : perennial plant of Europe and America having racemes of
         white or purplish flowers and intensely bitter trifoliate
         leaves; often rooting at water margin and spreading
         across the surface [syn: {water shamrock}, {buckbean}, {bogbean},
          {bog myrtle}, {marsh trefoil}]