The Collaborative International Dictionary of English v.0.48:

Operculum \O*per"cu*lum\, n.; pl. L. Opercula, E.
   Operculums. [L., a cover or lid, fr. operire to cover.]
   1. (Bot.)
      (a) The lid of a pitcherform leaf.
      (b) The lid of the urnlike capsule of mosses.
          [1913 Webster]

   2. (Anat.)
      (a) Any lidlike or operculiform process or part; as, the
          opercula of a dental follicle.
      (b) The fold of integument, usually supported by bony
          plates, which protects the gills of most fishes and
          some amphibians; the gill cover; the gill lid.
      (c) The principal opercular bone in the upper and
          posterior part of the gill cover.
          [1913 Webster]

   3. (Zool.)
      (a) The lid closing the aperture of various species of
          shells, as the common whelk. See Illust. of
          Gastropoda.
      (b) Any lid-shaped structure closing the aperture of a
          tube or shell.
          [1913 Webster]

WordNet (r) 3.0 (2006):

operculum
    n 1: a hard flap serving as a cover for (a) the gill slits in
         fishes or (b) the opening of the shell in certain
         gastropods when the body is retracted