flashback vs flash back

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

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

flashback (noun)	flash back (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)

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