Definitions of reef:

• noun:   a submerged ridge of rock or coral near the surface of the water
• noun:   a rocky region in the southern Transvaal in northeastern South Africa; contains rich gold deposits and coal and manganese
• 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

  Example: "Reef the sailboat's mast"

• name:  A surname (very rare: popularity rank in the U.S.: #31274)