Definition of reef:
a submerged ridge of rock or coral near the surface of the water
a rocky region in the southern Transvaal in northeastern South Africa; contains rich gold deposits and coal and manganese
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
lower and bring partially inboard; "reef the sailboat's mast"
roll up (a portion of a sail) in order to reduce its area
reduce (a sail) by taking in a reef