What is the name given to an almost circular coral reef inside which there is a lagoon?

An atoll is a coral reef structure characterized by its circular or semi-circular shape that encircles a central lagoon [4]. According to the Darwinian theory of reef formation, atolls typically develop when a fringing reef forms around a volcanic island; as the island subsides or sinks completely below sea level, the coral continues to grow upward, leaving a ring-shaped reef surrounding a body of water [t1]. Unlike fringing reefs, which are contiguous with the shore, or barrier reefs, which run parallel to the coastline separated by a lagoon, atolls enclose a shallow lagoon without any central land mass [c2][c4]. These formations are primarily found in tropical regions of the Pacific and Indian Oceans [t4]. While fringing and barrier reefs are also associated with lagoons, only the atoll is defined by its almost circular ring-like structure enclosing the water [c4][t2].

Sources

1. [2] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 11: Islands and Coral Reefs > Types of Goral Reef > p. 100
2. [4] https://oceanservice.noaa.gov/education/tutorial_corals/coral04_reefs.html