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):

Vaccinium caespitosum
    n 1: low-growing tufted deciduous shrub of northern and alpine
         North America having pink to coral-red flowers followed by
         sweet blue berries [syn: dwarf bilberry, dwarf
         blueberry, Vaccinium caespitosum]