The Collaborative International Dictionary of English v.0.48:

Labium \La"bi*um\, n.; pl. L. Labia, E. Labiums. [L.]
   1. A lip, or liplike organ.
      [1913 Webster]

   2. The lip of an organ pipe.
      [1913 Webster]

   3. pl. (Anat.) The folds of integument at the opening of the
      vulva.
      [1913 Webster]

   4. (Zool.)
      (a) The organ of insects which covers the mouth beneath,
          and serves as an under lip. It consists of the second
          pair of maxill[ae], usually closely united in the
          middle line, but bearing a pair of palpi in most
          insects. It often consists of a thin anterior part
          (ligula or palpiger) and a firmer posterior plate
          (mentum).
      (b) Inner margin of the aperture of a shell.
          [1913 Webster]

WordNet (r) 3.0 (2006):

labium
    n 1: a liplike structure that bounds a bodily orifice
         (especially any of the four labiate folds of a woman's
         vulva)