turn back vs relapse

turn back is not a noun while relapse is a noun.

Word	Noun	Adjective	Verb	Adverb
turn back	No	No	Yes	No
relapse	Yes	No	Yes	No

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

Other hyponyms of turn back include fall back, lapse, recidivate, regress, retrogress, resile, go back, recover, recuperate.

turn back (noun)	relapse (noun)
	a failure to maintain a higher state

turn back (verb)	relapse (verb)
go back to a previous state	go back to bad behavior
turn inside out or upside down	deteriorate in health
hold back, as of a danger or an enemy; check the expansion or influence of
retrace one's course
force to go away; used both with concrete and metaphoric meanings