Atoll

Etymology

From Dhivehi "atoḷu," the language of the Maldives, whose ring-shaped reefs are the classic example. The word entered wide English use after Charles Darwin adopted it in his work on coral reefs.

noun · /AT-ol/
A ring-shaped coral reef or chain of coral islands enclosing a shallow lagoon.
What it names
A distinctive island form — a coral ring around a lagoon — with an origin outside the European languages.
Example
The plane descended toward a green atoll ringing a turquoise lagoon.
Related idea
The still water an atoll encloses is a lagoon; the ring of coral is the atoll itself.