The Collaborative International Dictionary of English v.0.48:

Lunule \Lu"nule\, n. [F., fr. L. lunula, dim. of luna moon.]
   1. (Anat.) Anything crescent-shaped; a crescent-shaped part
      or mark; a lunula; a lune.
      [1913 Webster]

   2. (Chem.) A lune. See Lune.
      [1913 Webster]

   3. (Zool.)
      (a) A small or narrow crescent.
      (b) A special area in front of the beak of many bivalve
          shells. It sometimes has the shape of a double
          crescent, but is oftener heart-shaped. See Illust. of
          Bivalve.
          [1913 Webster]

WordNet (r) 3.0 (2006):

lunule
    n 1: the crescent-shaped area at the base of the human
         fingernail [syn: half-moon, lunula, lunule]