The Collaborative International Dictionary of English v.0.48:

Bilberry \Bil"ber*ry\, n.; pl. Bilberries. [Cf. Dan.
   b["o]lleb[ae]r bilberry, where b["o]lle is perh. akin to E.
   ball.]
   1. (Bot.) The European whortleberry (Vaccinium myrtillus);
      also, its edible bluish black fruit.
      [1913 Webster]

            There pinch the maids as blue as bilberry. --Shak.
      [1913 Webster]

   2. (Bot.) Any similar plant or its fruit; esp., in America,
      the species Vaccinium myrtilloides, Vaccinium
      c[ae]spitosum and Vaccinium uliginosum.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bilberry
    n 1: erect European blueberry having solitary flowers and blue-
         black berries [syn: bilberry, whortleberry,
         whinberry, blaeberry, Viccinium myrtillus]
    2: erect blueberry of western United States having solitary
       flowers and somewhat sour berries [syn: bilberry, thin-
       leaved bilberry, mountain blue berry, Viccinium
       membranaceum]
    3: blue-black berries similar to American blueberries [syn:
       bilberry, whortleberry, European blueberry]