What does "hair space" mean?

A piece of metal type used to create the narrowest space.

What is a synonym for "hair space"?

Common synonyms include: ace, bit, caesura, crack, discontinuity, earshot.

noun

noun ·Rare ·Advanced level

Noun

1
A piece of metal type used to create the narrowest space.
2
(printing) the narrowest of the spaces used to separate words or letters wordnet
3
The narrowest space appearing between letters and punctuation. US

Referring to the thinness of a hair.