The Collaborative International Dictionary of English v.0.48:

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 Rhamnus catharticus
   are used in medicine. The latter is used for hedges.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Sea buckthorn, a plant of the genus Hippopha["e].
      [1913 Webster]

WordNet (r) 3.0 (2006):

Rhamnus
    n 1: type genus of the Rhamnaceae: buckthorns [syn: Rhamnus,
         genus Rhamnus]