From The Collaborative International Dictionary of English v.0.44 [gcide]:
Operculum \O*per"cu*lum\, noun; 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.
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.
3. (Zo["o]l.) (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.
From WordNet (r) 2.0 [wn]:
noun
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 [also: {opercula} (pl)]
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