Sometimes refered to as a life preserver, a flotation device is a piece of emergency equipment designed to keep one from drowning. Sometimes used in pools by lifeguards, or on sinking ships.

The seats in many airplanes have cushions that can be used to float in water. They have straps on the underside, for the user to hold onto. The airline safety video they make passengers watch typically states, "In the event of a water landing, the cushion under your seat can be used as a flotation device."

I believe a life preserver refers more specifically to those orange things you wear around your neck, while "flotation device" suggests a less user-friendly lifesaver.