rapid eye movement vs REM sleep

rapid eye movement and REM sleep both are nouns.

Word	Noun	Adjective	Verb	Adverb
rapid eye movement	Yes	No	No	No
REM sleep	Yes	No	No	No

As nouns, rapid eye movement and REM sleep are synonyms defined as:

rapid eye movement and REM sleep: a recurring sleep state during which dreaming occurs; a state of rapidly shifting eye movements during sleep

Other synonyms of rapid eye movement include paradoxical sleep, rapid eye movement sleep, REM.

rapid eye movement (noun)	REM sleep (noun)
a recurring sleep state during which dreaming occurs; a state of rapidly shifting eye movements during sleep	a recurring sleep state during which dreaming occurs; a state of rapidly shifting eye movements during sleep