agnetic) See electromagnetic radiation
In typography, a unit of measure equal to the width of the capital letter "M" in a particular font. Ems are widely used in website typography as a basic unit of measure. Rather than using pixels, which is a fixed width on any screen, the default font size is set to ems, and all fonts on the page that are larger or smaller are designated as multiples or fractions of ems.
Em Dash, En Dash
The terms "em" and "en" are the dashes commonly found in typography, but which are not standard keyboard keys. An "em dash" (one em wide) is used to convey changes such as an abrupt break in thought, a digression or a change from one speaker to another. "En dashes" (width of letter N) are shorter and are used for ranges such as numbers and dates. Contrast with the hyphen, which is a regular text character used to separate telephone and account numbers.
Em and En Dashes in HTML
The mdash and ndash are the codes used to create em and en dashes on HTML pages, whereas the hyphen is a regular text character in HTML.