1. noun /riːf/
a) A chain or range of rocks, sand, or coral lying at or near the surface of the water.
b) & (South Africa) A large vein of auriferous quartz; hence, any body of rock yielding valuable ore.
2. verb /riːf/
a) To take in part of a sail in order to adapt the size of the sail to the force of the wind.
b) To pull or yank strongly "He reefed the horses head around sharply."
Wikipedia foundation.