The Collaborative International Dictionary of English v.0.48:

Indention \In*den"tion\, n. (Print.)
   Same as Indentation, 4.
   [1913 Webster]

WordNet (r) 3.0 (2006):

indention
    n 1: the space left between the margin and the start of an
         indented line [syn: indentation, indention, indent,
         indenture]