Glossary Term Definition
A unit of measurement equal to the current type size; in 12 pt type, one em is 12 points. Used as the basis for the em dash, em space, and other proportional spacing units.
Definition
A unit of measurement equal to the current type size. In 12 pt type, one em is 12 points. The em is the basis for the em dash and em space and serves as a fundamental reference unit throughout typographic measurement. Historically the em was the width of the letter M, but in modern usage it simply equals the type size.
Source
No access — Robert Bringhurst — p. 324:
A unit of measurement equal to the current type size.