An em is a unit of measurement in the field of typography. The unit is defined in the terms of a specific typeface, and thus varies in length.
1 em is sometimes said to be equal to the width of a capital "M" in a particular typeface. However, as the term has expanded to include a wider variety of languages and character sets, its meaning has evolved; this has allowed it to include those fonts, typefaces, and character sets which do not include a capital "M", such as Chinese and the Arabic alphabet. In modern usage, em has changed from a strictly horizontal measurement to a much more powerful and versatile one based on the standard measurements of a given font, both horizontal and vertical.
In current use, em usually means the typeface's body size, meaning the length from the lowest descender to the highest ascender, sometimes including height added by any diacritical marks. So, 1 em in a 16 pt typeface is 16 points. This length is also called a firet.
An "em-quad" is a metal spacer used in printing presses. It is referred to by this name because it is composed of a square one em on each side. In these old-fashioned printing presses, this allowed the insertion of an em space ( ) character between other typographical characters.
The width of the em space ( ) is defined to be 1 em, as is the em dash (—) (more commonly used in American texts). By contrast, the narrower unit en (more commonly found in European texts) is ½ em.
Online, the use of the em measurement has become more common; with the development of Cascading Style Sheets (or CSS), the W3C best practises recommendations within HTML and online markup now call for web pages to be based on scalable designs, using a relative unit of measurement (such as the em measurement), rather than a fixed one such as point size.