rattlesnake vs pit viper

rattlesnake and pit viper both are nouns.

Word	Noun	Adjective	Verb	Adverb
rattlesnake	Yes	No	No	No
pit viper	Yes	No	No	No

As nouns, pit viper is a hypernym of rattlesnake; that is, pit viper is a word with a broader meaning than rattlesnake:

rattlesnake: pit viper with horny segments at the end of the tail that rattle when shaken
pit viper: New World vipers with hollow fangs and a heat-sensitive pit on each side of the head

rattlesnake (noun)	pit viper (noun)
pit viper with horny segments at the end of the tail that rattle when shaken	New World vipers with hollow fangs and a heat-sensitive pit on each side of the head