common dolphin vs dolphin

common dolphin and dolphin both are nouns.

Word	Noun	Adjective	Verb	Adverb
common dolphin	Yes	No	No	No
dolphin	Yes	No	No	No

As nouns, dolphin is a hypernym of common dolphin; that is, dolphin is a word with a broader meaning than common dolphin:

common dolphin: black-and-white dolphin that leaps high out of the water
dolphin: any of various small toothed whales with a beaklike snout; larger than porpoises

common dolphin (noun)	dolphin (noun)
black-and-white dolphin that leaps high out of the water	any of various small toothed whales with a beaklike snout; larger than porpoises
	large slender food and game fish widely distributed in warm seas (especially around Hawaii)