EM
WordNet
noun
(1) A linear unit (1/6 inch) used in printing
(2) The square of a body of any size of type
WiktionaryText
Etymology
The typographic em is named after the metal type for the capital in early printing, whose body was square (the printed letter M is almost never one em in width).
Noun
- The name of the letter M, m. Found in compounds such as emcee.
- The ems and ens at the beginnings and ends.
- A unit of measurement equal to the height of the type in use.