There are 2 definitions of em in English:

em1

Syllabification: em

noun

Printing
1A unit for measuring the width of printed matter, equal to the height of the type size being used.
More example sentences
  • From there, sizing descendant elements with ems (a relative unit of measurement) becomes much more logical: 1em is 10 px, 1.6em is 16 px, .9em is 9px, and so forth.
  • Sterne was almost compulsively attentive to page layout and typography, going so far as to specify the length of each dash in printer's ems.
  • It doesn't matter which unit you use - zero horseshoes is the same as zero pixels, ems, or percentages.
1.1A unit of measurement equal to twelve points.
More example sentences
  • So, a font size of 1.25 ems will be displayed as 1.25 pixels.
  • This reduced the text to an unreadable level when the CSS specified sizes in ems, but only reduced it slightly when specified as percentages.

Origin

late 18th century: the letter M represented as a word, since it is approximately this width.

Definition of em in:

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Pronunciation: vɪˈtjuːpəreɪt
verb
blame or insult (someone) in strong language...

There are 2 definitions of em in English:

EM2

Syllabification: EM

abbreviation

1Electromagnetic.
1.1Engineer of Mines.
1.2Enlisted man (men).