Source : Webster's Revised Unabridged Dictionary (1913)

Buckthorn \Buck"thorn`\, n. (Bot.)
   A genus ({Rhamnus}) of shrubs or trees. The shorter branches
   of some species terminate in long spines or thorns. See
   {Rhamnus}.

   {Sea buckthorn}, a plant of the genus {Hippopha["e]}.

Source : WordNet®

buckthorn
     n 1: a shrub or shrubby tree of the genus Rhamnus; fruits are
          source of yellow dyes or pigments
     2: any shrub or small tree of the genus Bumelia
     3: an Old World plantain with long narrow ribbed leaves widely
        established in temperate regions [syn: {English plantain},
         {narrow-leaved plantain}, {ribgrass}, {ribwort}, {ripple-grass},
         {Plantago lanceolata}]