Wiktionary
n. 1 (context metal type English) A metal block used to separate words, one fifth of an em in width. 2 (context typography English) In digital text, a character representing a thin space.
Wikipedia
In typography, a thin space is a space character that is usually or of an em in width. It is used to add a narrow space, such as between nested quotation marks or to separate glyphs that interfere with one another. It is not as narrow as the hair space.
In Unicode, thin space is encoded at . Unicode's is a non-breaking space with a width similar to that of the thin space.
In LaTeX and Plain TeX, <kbd>\thinspace</kbd> produces a narrow, non-breaking space. Outside math formulas in LaTeX, <kbd>\,</kbd> also produces a narrow, non-breaking space, but inside math formulas it produces a narrow, breakable space.
In some versions of Microsoft Word, the symbol dialog (often available via Insert > Symbol or Insert > Special Characters), has both the thin space and the narrow no-break space available for point-and-click insertion. In Word's Symbol dialog, under font = "(normal text)", they are found in subset = "General Punctuation", Unicode character 2009 and nearby. Other word processing programs have ways of producing a thin space.
Usage examples of "thin space".
For eight days they had been struggling across the blistering shifting sands, walking by night, sweltering under the thin space cloth during the day.
A piece of thin space cloth had been stretched between the two ships to shield the men from the blazing sun.
I got down on my hands and knees in front of the Sotos' door and looked through the thin space between the door bottom and the Ca-brillo Building's threadbare carpeting.
Sometimes rupert thought he could see a faint stain of yellow on the crystal-clear glass from another pool of Mies' urine, after the boy had blindly chased a ball onto it, then frozen in fright, seemingly floating in thin space above the front door.
I had to sieve in around the door, using the extremely thin space between its dense metal and the jamb.
It filled the whisper-thin space between his shoulder and Clemantine's, between his shoulder and Urban's.
Isaac reached awkwardly into the thin space in front of the cages and swung the great window open.
And where the plasteel cutter had shorted out, a milky gray gas clouded the thin space over the water.