dolphin vs killer whale

dolphin and killer whale both are nouns.

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

As nouns, killer whale is a hyponym of dolphin; that is, killer whale is a word with a more specific, narrower meaning than dolphin:

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

dolphin (noun)	killer whale (noun)
any of various small toothed whales with a beaklike snout; larger than porpoises	predatory black-and-white toothed whale with large dorsal fin; common in cold seas
large slender food and game fish widely distributed in warm seas (especially around Hawaii)