Glossary Term Definition
The thinnest fixed space in traditional typography, typically one-fifth or one-sixth of an em, used for fine-tuning letter and punctuation spacing.
Definition
The thinnest fixed space in traditional typography, typically one-fifth or one-sixth of an em. Hair spaces are used for fine-tuning letter and punctuation spacing — for example, adding a sliver of space around an em dash or between nested quotation marks.
Source
No access — Robert Bringhurst — p. 325:
The thinnest fixed space in traditional typography.