Real Dictionary

rapid eye movement in British English 1
noun: movement of the eyeballs under closed eyelids during paradoxical sleep, which occurs while the sleeper is dreaming

REM
REM sleep is a period of sleep that is very deep, during which your eyes and muscles make many small movements. It is the period during which most of your dreams occur. REM is an abbreviation for 'rapid eye movement'.

REM in American English
a basic unit used to measure the amount of biological damage caused by various types of ionizing radiation, equal to the dose that produces the same amount of damage in human tissue as one roentgen of X-rays or gamma rays (0.01 sievert)
the periodic, rapid, jerky movement of the eyeballs under closed lids during stages of sleep associated with dreaming
noun
noun: the quantity of ionizing radiation whose biological effect is equal to that produced by one roentgen of x-rays

REM in British English 1
abbreviation for: rapid eye movement

REM in British English 2
noun acronym for: roentgen equivalent man

roentgen equivalent man in British English 2
noun: the dose of ionizing radiation that produces the same effect in man as one roentgen of x- or gamma-radiation