hair space

from The American Heritage® Dictionary of the English Language, 5th Edition.

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

from The Century Dictionary.

• noun The thinnest space used by printers.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

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

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