Source : Webster's Revised Unabridged Dictionary (1913)

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

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

   3. A recess or sharp depression in any surface.

   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.

   {Hanging}, or {Reverse}, {indentation}, indentation of all
      the lines of a paragraph except the first, which is a full
      line.

Source : WordNet®

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