An atoll (// or //) is a ring-shaped coral reef including a coral rim that encircles a lagoon partially or completely. There may be coral islands/cays on the coral rim.
The word atoll comes from the Dhivehi (an Indo-Aryan language spoken on the Maldive Islands) word atholhu (Dhivehi: އަތޮޅު, [ˈət̪ɔɭu]). Its first recorded use in English was in 1625 as atollon – Charles Darwin recognized its indigenous origin and coined, in his The Structure and Distribution of Coral Reefs, the definition of atolls as "circular groups of coral islets" that is synonymous with "lagoon-island".
More modern definitions of atoll describe them as "annular reefs enclosing a lagoon in which there are no promontories other than reefs and islets composed of reef detritus" or "in an exclusively morphological sense, [as] a ring-shaped ribbon reef enclosing a lagoon".