flash back vs flashback

flash back is not a noun while flashback is a noun.

Word	Noun	Adjective	Verb	Adverb
flash back	No	No	Yes	No
flashback	Yes	No	No	No

flash back (noun)	flashback (noun)
	a transition (in literary or theatrical works or films) to an earlier event or scene that interrupts the normal chronological development of the story
	an unexpected but vivid recurrence of a past experience (especially a recurrence of the effects of an hallucinogenic drug taken much earlier)

flash back (verb)	flashback (verb)
return in time