fall back vs lapse

fall back is not a noun while lapse is a noun.

Word	Noun	Adjective	Verb	Adverb
fall back	No	No	Yes	No
lapse	Yes	No	Yes	No

As verbs, fall back and lapse are synonyms defined as:

Other synonyms of fall back include recidivate, regress, relapse, retrogress.

fall back (noun)	lapse (noun)
	a mistake resulting from inattention
	a failure to maintain a higher state
	a break or intermission in the occurrence of something

fall back (verb)	lapse (verb)
go back to bad behavior	go back to bad behavior
retreat	drop to a lower level, as in one's morals or standards
hang (back) or fall (behind) in movement, progress, development, etc.	end, at least for a long time
fall backwards and down	for time to move forward
move back and away from	pass into a specified state or condition; sink into
have recourse to	let slip