Wiktionary
punctuation space
n. (context in dated typography English) A (l en space) of non-(l en variable) width: ⊣ ⊢ equal to the width of a (l en period) ((l en full stop)) or (l en comma), inserted after ⟨“⟩ or ⟨‘⟩ and before ⟨”⟩ or ⟨’⟩ (and sometimes ⟨?⟩ or ⟨!⟩), unless a (l en punctuation mark) occurs there. It is Unicode character U+2008. When using X11 input method, the conventional key combination is ''Compose''+''Space''+''Period''.