tail fin also tail·fin 
(tālfĭn′)
(tālfĭn′)n.
1. A fin at the posterior part of the body of a fish, crustacean, whale, or other aquatic animal.
2. An ornamental projection shaped like a fin on the rear fender of an automobile. Also called fin1.
3. A fin at the tail end of an aerodynamic or hydrodynamic object such as an airplane or surfboard, used to provide directional control or stability.
tail fin1959 Cadillac Fleetwood Sixty Special