A membranous appendage extending from the body of a fish or other aquatic animal, used for propelling, steering, or balancing the body in the water. Something resembling a fin in shape or function, as: A fixed or movable airfoil used to stabilize an aircraft, missile, or projectile in flight. A thin, usually curved projection attached to the rear bottom of a surfboard for stability. A projecting vane used for cooling, as on a radiator or an engine cylinder. See tail fin. See flipper. To equip with fins. To emerge with the fins above water. To swim, as a fish. To lash the water with the fins. Used of a dying whale. Slang A five-dollar bill.