An island consisting of a circular coral reef surrounding a lagoon WordNet 3.0 & Open English WordNet

An atoll is a ring-shaped island, including a coral rim that encircles a lagoon. Wikipedia

Type of island. Wikipedia Disambiguation

Ring-shaped coral reef, generally formed over a former oceanic volcano, frequently with small emerging islands over the rim, often enclosing a central lagoon possibly still containing one or a few emerging elevated islands Wikidata

An 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. OmegaWiki

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 not all islands with a reef and a lagoon are atolls. Wiktionary

Island. Wiktionary (translation)