What is a ring-shaped coral reef with a lagoon in the center called?

A ring-shaped coral reef with a lagoon in the center is specifically referred to as an atoll. This type of reef typically forms around the rim of a submerged volcanic island, where coral grows in a circular pattern, creating a lagoon in the middle. The formation of atolls begins with the accumulation of coral reefs, which expand outward while the island erodes over time and eventually sinks.

This process leads to the characteristic structure of an atoll, distinguishing it from other types of reefs, such as barrier reefs, which run parallel to the shoreline but are separated by deeper water, and fringing reefs, which grow directly from the shore of an island or continent. The term "island reef" is not commonly used in scientific literature, making atoll the correct and most widely accepted term to describe this specific geological formation.