fall back vs ebb

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

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

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

Other hyponyms of fall back include ebb away, ebb down, ebb off, ebb out.

fall back (noun)	ebb (noun)
	the outward flow of the tide
	a gradual decline (in size or strength or power or number)

fall back (verb)	ebb (verb)
go back to bad behavior	fall away or decline
retreat	hem in fish with stakes and nets so as to prevent them from going back into the sea with the ebb
hang (back) or fall (behind) in movement, progress, development, etc.	flow back or recede
fall backwards and down
move back and away from
have recourse to