Source : Webster's Revised Unabridged Dictionary (1913)

Blackthorn \Black"thorn`\, n. (Bot.)
      (a) A spreading thorny shrub or small tree ({Prunus
          spinosa}), with blackish bark, and bearing little
          black plums, which are called sloes; the sloe.
      (b) A species of {Crat[ae]gus} or hawthorn ({C.
          tomentosa}). Both are used for hedges.