Em

• POS: Noun

CIDE DICTIONARY

Em, n. 

 The portion of a line formerly occupied by the letter m, then a square type, used as a unit by which to measure the amount of printed matter on a page; the square of the body of a type.  [1913 Webster]

OXFORD DICTIONARY

Em, n. Printing 1 a unit for measuring the amount of printed matter in a line, usually equal to the nominal width of capital M.
2 a unit of measurement equal to 12 points.

Idiom

em rule (or dash) a long dash used in punctuation.

Etymology

name of the letter M