means A ridge of rock, sand, or coral lying at or near the surface of the water, often forming a hazard to ships — or, specifically, the vast living structure built by colonies of coral polyps.

from From Old Norse 'rif,' meaning a rib or ridge — the same root that gives us 'rib' itself, picturing the reef as a bone-like ridge breaking the sea's surface. Norse sailors who named these hazards knew them as ship-killers long before anyone thought of them as living wonders. (A separate 'reef' — the part of a sail you roll up to shorten it — comes from the same Norse 'rif,' the idea of a ridge or fold, which is why sailors 'take in a reef' in heavy wind.)