Definition of reef

i. A ridgelike or moundlike structure, layered or massive, built by sedentary calcareous organisms, esp. corals, and consisting mostly of their remains; it is wave-resistant and stands above the surrounding sediment. Also, such a structure built in the geologic past and now enclosed in rock, commonly of differing lithology.

Ref: AGI

ii. A narrow ridge or chain of rocks either at the water surface or too shallow to permit safe passage of a vessel.

Compare with: bank

iii. A provincial term for a metalliferous mineral deposit, esp. gold bearing quartz.

See Also: reefing