The Collaborative International Dictionary of English v.0.48:

buckbean \buckbean\, Buck bean \Buck" bean`\(b[=e]n`). (Bot.)
   A perennial plant (Menyanthes trifoliata) of Europe and
   America which grows in moist and boggy places, having racemes
   of white, reddish, or purplish flowers and intensely bitter
   trifoliate leaves, sometimes used in medicine; marsh trefoil;
   -- called also bog bean. It often roots at the water margin
   and spreads across the surface.

   Syn: water shamrock, bogbean, bog myrtle, marsh trefoil,
        Menyanthes trifoliata.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

buckbean
    n 1: 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, Menyanthes trifoliata]