From The Collaborative International Dictionary of English v.0.44 [gcide]:
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.
From WordNet (r) 2.0 [wn]:
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