A coral atoll.
The definition of an atoll is a ring shaped coral reef, or close coral islands that enclose or almost enclose a lagoon.
The Bikini in the Pacific Ocean is an example of an atoll.
a ring-shaped coral island nearly or completely surrounding a lagoon
Origin of atollfrom atolu, in Maldive Islands dialect, dialectal of Malayalam
An island or chain of islets connected by a coral reef that nearly or entirely encloses a lagoon.
Origin of atollDhivehi atholhu probably akin to Sinhalese ätul interior (from the interior lagoon) probably from Sanskrit antala- interior variant of antara- ; see en in Indo-European roots.
An atoll is created as a coral reef builds up around an eroding volcanic island.