tail fin

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fin at the posterior part of the body of a fish, crustacean, whale, or other aquatic animal.
• noun An ornamental projection shaped like a fin on the rear fender of an automobile.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Alternative spelling of tailfin.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the tail of fishes and some other aquatic vertebrates
• noun a stabilizer that is part of the vertical tail structure of an airplane
• noun one of a pair of decorations projecting above the rear fenders of an automobile