Glossary Term Definition
A unit of measurement equal to half an em; used as the basis for the en dash and en space.
Definition
A unit of measurement equal to half an em. The en is used as the basis for the en dash and en space. In 12 pt type, one en is 6 points.
Source
No access — Robert Bringhurst — p. 324:
A unit of measurement equal to half an em.