1. [n]  (Roentgen Equivalent Man) the dosage of ionizing radiation that will cause the same amount of injury to human tissue as 1 roentgen of X-rays
2. [n]  a recurring sleep state during which dreaming occurs; a state of Rapid Eye Movements during sleep

REM is a 3 letter word that starts with R.

(From "remark") The keyword used in basic to introduce a comment (which continues to the end of the line). ms-dos probably borrowed it from basic. Might be used in the form "REM out" meaning to comment out.