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.
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.
(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 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.