A*toll" (&?;), n. [The native name in the Indian Ocean.] A coral island or islands, consisting of a belt of coral reef, partly submerged, surrounding a central lagoon or depression; a lagoon island.

• A type of island consisting of a ribbon reef that nearly or entirely surrounds a lagoon and supports, in most cases, one to many islets on the reef platform. Atolls have a unique geology, so all islands with a reef and a lagoon are not atolls.