Buoy (noun)

1. A floating marker anchored in a body of water to indicate a location or to warn of a hazard.
2. A device used as a marker or support on water.
3. A thing that serves as a warning or encouragement.

Origin:

From middle english boye, from old french boie, from latin boia, from boius ("cable"), from gaulish (compare welsh boyw).

Examples:

1. The lifeguard placed a buoy in the water to mark the safe swimming area.
2. The buoy was the only thing visible on the horizon.
3. The buoy's bright colors made it easy to spot.
4. The buoyancy of the buoy kept it afloat.
5. The buoy served as a marker for the boats to follow.