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.

Source : WordNet®

blackthorn
     n 1: a thorny Eurasian bush with plumlike fruits [syn: {sloe}, {Prunus
          spinosa}]
     2: erect and almost thornless American hawthorn with somewhat
        pear-shaped berries [syn: {pear haw}, {pear hawthorn}, {Crataegus
        calpodendron}, {Crataegus tomentosa}]