Definition of em in English:

em

noun

Printing
  • 1A unit for measuring the width of printed matter, equal to the height of the type size being used.

    • ‘It doesn't matter which unit you use - zero horseshoes is the same as zero pixels, ems, or percentages.’
    • ‘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.’
    • ‘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.’
    1. 1.1 A unit of measurement equal to twelve points.
      • ‘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.

Pronunciation:

em

/ɛm/