Pronunciation: (buk'thôrn"),[key] —n. 1. any of several, sometimes thorny trees or shrubs belonging to the genus Rhamnus, esp. R. frangula, the bark of which is used in medicine. Cf. buckthorn family. 2. a tree or shrub belonging to the genus Bumelia, of the sapodilla family, esp. B. lycioides, a thorny tree having elliptic leaves and large clusters of white flowers, common in the southern and part of the central U.S.