PFD

From Kayak Wiki

Personal Floation Device. Commonly called life jacket, but more properly called a bouyancy aid. I don't care how good a swimmer you are, if you paddle with out one you are asking to drown. Always wear and never take off if you are on whitewater. Some times made out of nylon and pvc is used for bouyancy.

It should be noted that this is not the case for flatwater sprint kayakers and canoers. In Canada the rule states that a PFD must be worn by anyone under the age of 14 (bantams and under). After that, a PFD is not required. In the United States, there are no rules regarding flotation devices for flatwater sprint kayaking.