Reef (noun)

1. A line of rocks or coral extending above or below the surface of the sea.
2. An underwater ridge or formation of coral or rock.
3. A barrier or obstruction to navigation.
4. A strip of material sewn across the width of a sail to reduce its area and keep it from becoming full of wind.

Origin:

From old english rif, from proto-germanic *ribjaz.

Examples:

1. The ship ran aground on a reef.
2. The coral reef is home to a diverse ecosystem.
3. The reef is a popular spot for diving and snorkeling.
4. The reef was a danger to ships sailing in the area.
5. The reef was used to reduce the sail area.