a flattened appendage of an aquatic animal, e.g. a fish or whale, used in propelling or guiding the body

a flattened projecting, often movable, part attached to the underside of a vessel, e.g. a submarine, to give it stability or to improve its controllability

a vertical aerofoil attached to an aircraft for directional stability

= flipper (2)

any of the projecting ribs on a radiator or an engine cylinder that improve heat transfer