Princeton's WordNet

1. reef(noun)

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

2. Witwatersrand, Rand, Reef(noun)

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

3. reef(verb)

   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

4. reef(verb)

   lower and bring partially inboard

5. reef(verb)

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

6. reef(verb)

   reduce (a sail) by taking in a reef