a. the square of any size of type used as the unit of measurement for matter printed in that type size b. (originally) the portion of a line of type occupied by the letter M in type of the same size
unit of measurement equal to about one-sixth of an inch (4 mm)
having the area of an em quad or the length of an em dash
gaseous product of radioactive disintegration, including radon, thoron, and actinon. Symbol: Em