Wiktionary
figure space
n. (context in fonts whose digits are of uniform width English) A (l en space) of non-(l en variable) width: ⊣ ⊢ equal to the width of a digit.
Wikipedia
Figure space
A figure space is a typographic unit equal to the size of a single typographic figure (numeral or letter), minus leading. Its size can fluctuate somewhat depending on which font is being used. This is the preferred space to use in numbers. It has the same width as a digit and keeps the number together for the purpose of line breaking.