operculum

plural opercula

1. biologyA covering flap in animals and insects, such as a gill cover or the lid of nits.

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

   2017 January 18, Susannah Lydon, The Guardian

   1 more example

   As the egg comes out, it gets covered in glue. All it needs now is the operculum, a little lid that will help your kid breathe.

   2023, translated in 2025, Berta Páramo and translated by Marc Correa Haro, Lice: How to Survive on Humans, page 110

2. biologyThe lidlike portion of a moss sporangium or of a fruit that detaches to allow the dispersal of spores or seeds.
3. dentistryA 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.

   1900 December – 1901 August, H[erbert] G[eorge] Wells, chapter IX, in The First Men in the Moon, London: George Newnes, […], published 1901, →OCLC, page 95

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