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.