Definition of "buckthorn"

Any of several, often thorny shrubs or small trees, especially

1. the family Rhamnaceae, the buckthorn family,

   1. especially genus Rhamnus,

      1. especially the common buckthorn or purging buckthorn; Rhamnus cathartica.  examples

   2. also genus Frangula, especially Frangula alnus, the alder buckthorn.  examples

2. Hippophae (family Elaeagnaceae), the sea buckthorn.  examples