affirm vs back up

affirm and back up both are verbs.

Word	Noun	Adjective	Verb	Adverb
affirm	No	No	Yes	No
back up	No	No	Yes	No

As verbs, back up is a hyponym of affirm; that is, back up is a word with a more specific, narrower meaning than affirm:

Other hyponyms of affirm include back, vouch, verify, demonstrate, establish, prove, shew, show, document, validate.

affirm (verb)	back up (verb)
establish or strengthen as with new evidence or facts	establish as valid or genuine
to declare or affirm solemnly and formally as true	become or cause to become obstructed
say yes to	make a copy of (a computer file) especially for storage in another place as a security copy
	move backwards from a certain position
	give moral or psychological support, aid, or courage to