En

Part of speech: noun

Definitions

1. A unit of horizontal measurement in typesetting equal to half the width of an em space
2. A printing measurement that equals half of an em, used to size spaces and dashes in type
3. The typographic unit of width, half the size of an em, applied to spacing and dash lengths

Etymology: Printers measured type by the em, the width of a capital M, and took half of that as a smaller unit. They named it after the narrower letter N, so the "en" is literally half an em, roughly the width of an N in the same typeface. The typographic sense is recorded from 1793. The en dash takes its name from this unit, since it is about as long as one en of space. The letter itself is the ordinary English name of N, and the printing trade simply borrowed that everyday letter name for its measurement.