operculum

noun

1. A covering flap in animals, such as a gill cover.

   “Hyoliths […] have a small, conical calcium carbonate shell, with a lid called an operculum.”

2. The lidlike portion of a moss sporangium or of a fruit that detaches to allow the dispersal of spores or seeds.
3. A gum flap covering (part of) a partially erupted tooth, usually a wisdom tooth.
4. A structure which serves as a cover or lid.

   “I lifted the circular operculum from its place and laid it carefully on the bale.”

5. One of several flaps of cerebral cortex covering the insula.