noun, plural ems.
the letter M, m.
Also called mut, mutton. Printing.
the square of any size of type used as the unit of measurement for matter printed in that type size.
(originally) the portion of a line of type occupied by the letter M in type of the same size.
Printing. having the area of an em quad or the length of an em dash.


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World English Dictionary
em (ɛm)
1.  mutton, Also called: mut the square of a body of any size of type, used as a unit of measurement
2.  pica em, Also called: pica a unit of measurement used in printing, equal to one sixth of an inch
[C19: from the name of the letter M]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

from Fr. assimilation of en- to following labial; also from L. ex-, assimilated to following -m-. Also a prefix used to form verbs from adjectives and nouns.

M.E., now taken as an abbreviation of them, but originally a form of hem, dative and accusative of the third person plural pronoun.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

EM abbr.
electron microscope

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Abbreviations & Acronyms
  1. electromagnetic

  2. electron microscope

  3. enlisted man

The American Heritage® Abbreviations Dictionary, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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