extremity vs fin

extremity and fin both are nouns.

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

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

extremity: an external body part that projects from the body
fin: organ of locomotion and balance in fishes and some other aquatic animals

Other hyponyms of extremity include chelicera, mouthpart, fang, chela, claw, nipper, pincer, parapodium, pleopod, swimmeret, limb, dactyl, digit.

extremity (noun)	fin (noun)
an external body part that projects from the body	organ of locomotion and balance in fishes and some other aquatic animals
that part of a limb that is farthest from the torso	a stabilizer on a ship that resembles the fin of a fish
the outermost or farthest region or point	one of a set of parallel slats in a door or window to admit air and reject rain
the greatest or utmost degree	a shoe for swimming; the paddle-like front is an aid in swimming (especially underwater)
an extreme condition or state (especially of adversity or disease)	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

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