The Collaborative International Dictionary of English v.0.48:

Cascara buckthorn \Cas"ca*ra buck"thorn`\ (Bot.)
   The buckthorn (Rhamnus Purshiana) of the Pacific coast of
   the United States, which yields cascara sagrada.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

cascara buckthorn
    n 1: shrubby tree of the Pacific coast of the United States;
         yields cascara sagrada [syn: cascara buckthorn,
         bearberry, bearwood, chittamwood, chittimwood,
         Rhamnus purshianus]