Etymology
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em (n.)

name of the letter M, c. 1200, from Latin; the Greek name was mu. In printing, originally the square corresponding in dimensions to the capital M of that type.

updated on August 14, 2014

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Definitions of em from WordNet

em (n.)
a quad with a square body;
since `em quad' is hard to distinguish from `en quad', printers sometimes called it a `mutton quad'
Synonyms: em quad / mutton quad
em (n.)
a linear unit (1/6 inch) used in printing;
Synonyms: pica em / pica
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.