atoll

/əˈtɒl/

IPA: /AE1 T AA0 L/

noun
  1. 1

    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

Source: https://en.wiktionary.org/wiki/atoll