replay vs play back

replay is a noun but play back is not a noun.

Word	Noun	Adjective	Verb	Adverb
replay	Yes	No	Yes	No
play back	No	No	Yes	No

As verbs, replay and play back are synonyms defined as:

replay (noun)	play back (noun)
the immediate rebroadcast of some action (especially sports action) that has been recorded on videotape
something (especially a game) that is played again

replay (verb)	play back (verb)
play again	reproduce (a recording) on a recorder
repeat a game against the same opponent
play (a melody) again
reproduce (a recording) on a recorder