| Definition -
(1) The name of the letter M.
(2) In Printing it labels a space the size of an m, thus an em dash is a dash the width of the letter m. Typographers use the em as the unit for measuring and estimating the amount of printed matter in a line, page, etc — the pica em is the standard.
Oxford English Dictionary - Its first OED citation is from 1793: "That em and en quadrats be included."
(Stower Printer's Gram. (1808) xvii. 419)