hair space

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. The narrowest of the metal spaces used for separating words or letters.
• n. A similar narrow space used in formatting text.

from Wiktionary, Creative Commons Attribution/Share-Alike License

• n. A piece of metal type used to create the narrowest space.
• n. The narrowest space appearing between letters and punctuation.

from the GNU version of the Collaborative International Dictionary of English

• n. the thinnest metal space used in lines of type.

from The Century Dictionary and Cyclopedia

• n. The thinnest space used by printers.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• n. (printing) the narrowest of the spaces used to separate words or letters