A "parenthetical dash" (the semantic meaning) can be typeset either as an em dash (a typographic meaning) without spaces, or as an en dash (a typographic meanin

A "parenthetical dash" (the semantic meaning) can be typeset either as an em dash (a typographic meaning) without spaces, or as an en dash (a typographic meaning) surrounded by spaces. And the latter is often referred to as an em dash (as a semantic meaning), since basically everyone uses that to mean "parenthetical dash" which is the correct term but a term that virtually nobody uses.