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reef:see coral reefcoral reefs,
limestone formations produced by living organisms, found in shallow, tropical marine waters. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone).
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a sharp underwater or above-water elevation in the sea floor in shallow water that hampers navigation. Reefs are formed either by the erosion of the bottom and coasts of the sea or by colonies of coral.
a device for reducing sail area during a strong wind. The reef is usually a series of bands threaded through the sail, by means of which the sail is folded up when the boom or yard is taken up—that is, the sail is reefed, or taken in. Sails may have from one to four reefs.