The Collaborative International Dictionary of English v.0.48:

Sloe \Sloe\ (sl[=o]), n. [OE. slo, AS. sl[=a]; akin to D. slee,
   G. schlehe, OHG. sl[=e]ha, Dan. slaaen, Sw. sl[*a]n, perhaps
   originally, that which blunts the teeth, or sets them on edge
   (cf. Slow); cf. Lith. slywa a plum, Russ. sliva.] (Bot.)
   A small, bitter, wild European plum, the fruit of the
   blackthorn (Prunus spinosa); also, the tree itself.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

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 (Crat[ae]gus
          tomentosa). Both are used for hedges.
          [1913 Webster]

WordNet (r) 3.0 (2006):

Prunus spinosa
    n 1: a thorny Eurasian bush with plumlike fruits [syn:
         blackthorn, sloe, Prunus spinosa]