an island consisting of a circular coral reef surrounding a lagoon
"The sailboat drifted through the calm waters inside the atoll, where the ring of white coral surrounded the hidden lagoon."
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
"The satellite imagery revealed the hidden structure of the remote island, showing it was an atoll rather than just any ring-shaped landmass because its coral reef completely encircled the deep central lagoon."