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.
Two branches
睡眠裡的快速動眼
這是三個字母被裝進去的第一個、也是最常見的內容：rapid eye movement。日常英文裡看到 REM，多半就是這一個。
noun · B2
REM is the part of sleep when the eyes move quickly from side to side behind closed eyelids, and when most dreaming happens. The three letters stand for rapid eye movement.
快速動眼期
樂團名字
這是專有名詞，用的是同樣三個字母。這個團為什麼取這個名字，這份資料裡沒有證據，所以這裡不做連結。
noun · B2
R.E.M. is the name of an American rock band that has been popular since the 1980s. Their albums include Automatic for the People and Monster.
R.E.M.（美國搖滾樂團）