rapid eye movement vs rapid eye movement sleep

rapid eye movement and rapid eye movement sleep both are nouns.

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

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

rapid eye movement and rapid eye movement 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, REM, REM sleep.

rapid eye movement (noun)	rapid eye movement 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