REEF, noun

A certain portion of a sail between the top or bottom and a row of eyelet holes, which is folded or rolled up to contract the sail, when the violence of the wind renders it necessary.

REEF, noun

A chain or range of rocks lying at or near the surface of the water.

REEF, verb transitive [from the noun.] To contract or reduce the extent of a sail by rolling or folding a certain portion of it and making it fast to the yard.