atoll
Definitions
A ring-shaped coral reef or string of closely spaced small coral islands that encloses or nearly encloses a lagoon.
A circular coral island formed by the growth of coral on the rim of a submerged volcanic crater.
Examples
The remote atoll is surrounded by crystal-clear turquoise waters and vibrant coral reefs.
After days of sailing across the open ocean, the small atoll finally appeared on the horizon.