indentation (n.1)

"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).

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Definitions of indentation from WordNet

indentation (n.)
a concave cut into a surface or edge (as in a coastline);
Synonyms: indenture
indentation (n.)
the formation of small pits in a surface as a consequence of corrosion;
Synonyms: pitting / roughness
indentation (n.)
the space left between the margin and the start of an indented line;
Synonyms: indention / indent / indenture
indentation (n.)
the act of cutting into an edge with toothlike notches or angular incisions;