A measure of distance equal to the body of a capital. In linear measure, a distance equal to the type size, and in square measure, the square of the type size. Thus an em is 12pt (or a 12pt square) in 12pt type. Also called mutton.
n. A typographical unit of measure that is equal in width to the point size of a particular font. For many fonts, this is equal to the width of a capital M, from which the em space takes its name. Compare en space, fixed space, thin space.