Source : Webster's Revised Unabridged Dictionary (1913)

Rhamnus \Rham"nus\, n. [NL., from Gr. "ra`mnos a kind of prickly
   shrub; cf. L. rhamnos.] (Bot.)
   A genus of shrubs and small trees; buckthorn. The California
   {Rhamnus Purshianus} and the European {R. catharticus} are
   used in medicine. The latter is used for hedges.

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®

Rhamnus
     n : type genus of the Rhamnaceae: buckthorns [syn: {genus
         Rhamnus}]