A lifebuoy is a life-saving device or life preserver. This term is most commonly used to refer to such a device when it is in the shape of a ring.
A lifebuoy is used to help prevent drowning by letting a person keep his head above water with minimal effort. A lifebuoy can also be fitted with a seawater-activated light to make it easier to spot during nighttime rescue efforts. Lifebuoys are part of the safety equipment required on many boats.
The lifebuoy is also known as the "Kisby Ring" or "Kisbie Ring" and is thought to have been given that name after British Naval Officer Thomas Kisbee.