killer whale vs dolphin

killer whale and dolphin both are nouns.

Word	Noun	Adjective	Verb	Adverb
killer whale	Yes	No	No	No
dolphin	Yes	No	No	No

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

killer whale: predatory black-and-white toothed whale with large dorsal fin; common in cold seas
dolphin: any of various small toothed whales with a beaklike snout; larger than porpoises

killer whale (noun)	dolphin (noun)
predatory black-and-white toothed whale with large dorsal fin; common in cold seas	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)