Definition of "Reef" :

noun: one of several strips across a sail that can be taken in or rolled up to lessen the area of the sail that is exposed to the wind

noun: a rocky region in the southern Transvaal in northeastern South Africa; contains rich gold deposits and coal and manganese

noun: a submerged ridge of rock or coral near the surface of the water

verb: reduce (a sail) by taking in a reef

verb: roll up (a portion of a sail) in order to reduce its area

verb: lower and bring partially inboard

"Reef the sailboat's mast."