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

The Free On-line Dictionary of Computing (19 January 2023):

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)