"cut, notch, incision," 1728, of margins or edges, extended form of indent (n.).
indentation n.2
"action of making a dent or impression; small hollow or depression, slight pit," 1847, from indent (v.2).
Entries linking to indentation
indent v.2
"to dent or press in," c. 1400, from in (adv.) + dent (v.). Etymologically distinct from indent (v.1) but felt as the same.
indent n.
"cut or notch in a margin," 1590s, from indent (v.1). A supposed earlier noun sense of "a written agreement" (late 15c.) is described in The Middle English Compendium as "scribal abbrev. of endenture."
indention n.
1763, formed irregularly from indent (v.1). It could be a useful word if it split with indentation the two senses (relating to marginal notches and to dents) of that word, but indention, too, is used in both.