reef (Noun) — A submerged ridge of rock or coral near the surface of the water.
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.
reef (Noun) — A rocky region in the southern Transvaal in northeastern South Africa; contains rich gold deposits and coal and manganese.
reef (Verb) — Lower and bring partially inboard. ex. "reef the sailing boat's mast"
reef (Verb) — Roll up (a portion of a sail) in order to reduce its area.
reef (Verb) — Reduce (a sail) by taking in a reef.