lifebuoy

A ring-shaped flotation device designed to be thrown to a person in the water to provide buoyancy and to prevent drowning. It differs from an innertube because it is often made of solid buoyant material (rather than being air-filled) and is colored to maximize visibility in the water. It also may have a provided line to pull the casualty holding onto it.

Note: The term "life buoy" is most common in British English. In American English these are better known as "life preservers".