The Collaborative International Dictionary of English v.0.48:

Indentation \In`den*ta"tion\, n.
   1. The act of indenting or state of being indented.
      [1913 Webster]

   2. A notch or recess, in the margin or border of anything;
      as, the indentations of a leaf, of the coast, etc.
      [1913 Webster]

   3. A recess or sharp depression in any surface.
      [1913 Webster]

   4. (Print.)
      (a) The act of beginning a line or series of lines at a
          little distance within the flush line of the column or
          page, as in the common way of beginning the first line
          of a paragraph.
      (b) The measure of the distance; as, an indentation of one
          em, or of two ems.
          [1913 Webster]

   Hanging indentation, or Reverse indentation, indentation
      of all the lines of a paragraph except the first, which is
      a full line; also called a hanging indent.
      [1913 Webster]

WordNet (r) 3.0 (2006):

indentation
    n 1: a concave cut into a surface or edge (as in a coastline)
         [syn: indentation, indenture]
    2: the formation of small pits in a surface as a consequence of
       corrosion [syn: pitting, roughness, indentation]
    3: the space left between the margin and the start of an
       indented line [syn: indentation, indention, indent,
       indenture]
    4: the act of cutting into an edge with toothlike notches or
       angular incisions

Moby Thesaurus II by Grady Ward, 1.0:

91 Moby Thesaurus words for "indentation":
   alveolation, alveolus, blaze, boss, bump, cleft, colophon,
   concavity, convexity, crena, cut, dactylogram, dactylograph, dent,
   depression, digging, dimple, dint, embossment, engraving,
   excavation, excrescence, feel, fingerprint, finish, footmark,
   footprint, footstep, fossil footprint, furrow, gash, gouge, grain,
   granular texture, hack, hollow, honeycomb, ichnite, ichnolite,
   impress, impression, imprint, incision, indent, indention,
   indenture, jag, jog, joggle, kerf, knub, lump, mark, mining, nap,
   nick, nock, notch, nub, pad, paw print, pawmark, pile, pimple, pit,
   pock, pockmark, print, protuberance, pug, pugmark, score, scotch,
   seal, shag, sigil, signet, stamp, step, structure, stud,
   sunken part, surface, surface texture, texture, thumbmark,
   thumbprint, vestige, wale, weave, woof



The Free On-line Dictionary of Computing (30 December 2018):

indentation
indent

    Space and/or tab characters added at the
   beginning of a line to indicate structure, e.g. indenting a
   quotation to make it stand out or indenting a block of code
   controlled by an if statement.

   Indentation is important in source code for readability.  There
   are a number of different indent styles.  Some programming
   languages go further and use indentation as the main method to
   represent block structure to the compiler or interpreter, see
   off-side rule.

   (2008-10-23)