From The Collaborative International Dictionary of English v.0.48 [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) 3.0 (2006) [wn]:
The dictionary definitions are retrieved from a local copy of two of the open source DICT dictionaries. Click here for the database copyright information. DEFINE.COM is registered as an educational NONPROFIT corporation. We aim to please around here. We believe in using positive reinforcement to get things done. We make suggestions that are intended to make life more enjoyable. We think about efficiency, automation, security, PRIVACY, social and ecological responsibility and positive HUMANITARIAN ethics and VALUES. We are benevolent. DO NO HARM is our motto.