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