Rem

Rem is a term with several distinct meanings in technology, science, and everyday use.

In web design, rem stands for root em and is a CSS length unit relative to the font size of the root element, typically the HTML element. If the root font size is 16px, 1rem equals 16px. Rems provide consistent sizing across a document, since they do not cascade from nested elements the way ems do. They are widely supported in modern browsers and are commonly used for typography, spacing, and component sizing to facilitate responsive and accessible design.

REM sleep refers to Rapid Eye Movement sleep, a distinct stage of the sleep cycle. During REM, eye movements are rapid, brain activity increases, and dreaming is common, while most voluntary muscles are relaxed. EEG patterns during REM resemble wakefulness. REM periods occur in cycles throughout the night, usually beginning after a period of non-REM sleep and lengthening in later cycles. REM sleep is associated with memory consolidation, learning, and mood regulation; disruptions can affect cognitive function and emotional well-being.

Other uses: REM historically stands for roentgen equivalent man, a dose‑of‑radiation unit used to quantify effective exposure. It has largely been replaced by the sievert (Sv), with 1 rem equal to 0.01 sievert. The rem unit remains encountered in older literature and some safety guidelines.