Space set at the start of a line to show it belongs under what came before. Structure shown by position alone.
Old French endenter (to notch, cut with teeth), from Latin dens (tooth); the legal sense of a jagged-cut duplicate document gave way to the plain sense of a notch, then a set-back space at a line's start.
Structure shown by where a line sits rather than by anything written on it — position alone doing the work of a label.