From the word’s history
Initialism of rapid eye movement.
rapid eye movement「快速動眼」的首字母縮寫。
You can think of rem as a three-letter shell rather than a word. Different fields have each pushed their own long phrase into the same three letters — rapid eye movement, roentgen equivalent in man, root em, remark. Nothing in the letters themselves carries meaning, so the way to read this one is to look at the words standing next to it and ask which long phrase has been packed inside.
rem 可以想成一個只有三個字母的空殼，而不是一個有意思的字。不同領域各自把自己的長詞塞進這三個字母裡：rapid eye movement、roentgen equivalent in man、root em、remark。字母本身不帶意思。所以讀它的方法不是拆字，而是看旁邊：旁邊是 sleep，就是睡眠；旁邊是數字和劑量，就是輻射單位；旁邊是樣式或程式碼，就是網頁單位或註解。
登入就可以看每一支怎麼連回核心畫面。