Definitions:
originally: any of various thorny shrubs, mostly of the family rhamnaceae. in later use: specifically any plant of the genus rhamnus (family rhamnaceae), comprising shrubs and small trees native chiefly to the northern hemisphere, and including the common or purging buckthorn, r. cathartica, alder-leaved buckthorn, r. alnifolia, and numerous other species; (in form rhamnus) the genus itself.