REM vs rapid eye movement sleep

REM and rapid eye movement sleep both are nouns.

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

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

REM 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 REM include paradoxical sleep, rapid eye movement, REM sleep.

REM (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