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A circular coral reef or string of coral islands surrounding a lagoon.
A coral island or series of coral islands forming a ring that nearly or entirely encloses a shallow lagoon. Atolls are surrounded by deep ocean water and range in diameter from about 1 km (0.62 mi) to over 100 km (62 mi). [A.H.]
1620s, atollon, from Malayalam atolu “reef,” probably from adal “closing, uniting.” Popularized in present form by Darwin’s writings. [A.H.]