fin vs tail fin

fin and tail fin both are nouns.

Word	Noun	Adjective	Verb	Adverb
fin	Yes	No	Yes	No
tail fin	Yes	No	No	No

As nouns, tail fin is a hyponym of fin; that is, tail fin is a word with a more specific, narrower meaning than fin:

fin: organ of locomotion and balance in fishes and some other aquatic animals
tail fin: the tail of fishes and some other aquatic vertebrates

Other hyponyms of fin include dorsal fin, pectoral fin, pelvic fin, ventral fin, caudal fin.

As nouns, fin and tail fin are synonyms defined as:

fin and tail fin: one of a pair of decorations projecting above the rear fenders of an automobile

Other synonyms of fin include tailfin.

fin (noun)	tail fin (noun)
organ of locomotion and balance in fishes and some other aquatic animals	the tail of fishes and some other aquatic vertebrates
a stabilizer on a ship that resembles the fin of a fish	a stabilizer that is part of the vertical tail structure of an airplane
one of a set of parallel slats in a door or window to admit air and reject rain	one of a pair of decorations projecting above the rear fenders of an automobile
a shoe for swimming; the paddle-like front is an aid in swimming (especially underwater)
one of a pair of decorations projecting above the rear fenders of an automobile
the cardinal number that is the sum of four and one

fin (verb)	tail fin (verb)
show the fins above the water while swimming
propel oneself through the water in a finning motion
equip (a car) with fins