em
UK[em]US[em]
n
The letter M, m.
n
A unit of measurement in printing, equal to the width of the letter M in a given font and size.
Etymology
The word 'em' is simply the spoken name for the letter M. The letter M itself originates from the Phoenician letter mem, meaning "water," which was adapted into the Greek mu and later the Latin M. The English name for the letter evolved naturally from its pronunciation in the Latin alphabet. As a typographical unit, the 'em' is a direct borrowing of this letter name into the specialized jargon of printing, where it serves as a fundamental measure of horizontal space based on the width of that specific character.
Analysis
This is a simplex word. It is the name of a letter of the English alphabet, derived from its pronunciation.
Examples
The word "mother" starts with the letter em.
In this font, an em dash is significantly wider than an en dash.
The typesetter specified a paragraph indent of one em.