a flotation device

A "flotation device" is a formal word for something that helps you float in the water.

A "personal flotation device" can also be called by other names, including "life jacket". It's a vest which you wear around the water to help you to float if you fall in. 

Another type of "flotation device" is a "lifesaver", which is a ring with a hole in the middle that you can throw to someone who's drowning in the water.

This phrase appears in these lessons: