Fin
WordNet
noun
(1) Organ of locomotion and balance in fishes and some other aquatic animals
(2) A stabilizer on a ship that resembles the fin of a fish
(3) A shoe for swimming; the paddle-like front is an aid in swimming (especially underwater)
(4) One of a set of parallel slats in a door or window to admit air and reject rain
(5) One of a pair of decorations projecting above the rear fenders of an automobile
(6) The cardinal number that is the sum of four and one
verb
(7) Show the fins above the water while swimming
"The sharks were finning near the surface"
(8) Propel oneself through the water in a finning motion
(9) Equip (a car) with fins