The Collaborative International Dictionary of English v.0.48:

Truncation \Trun*ca"tion\, n. [L. truncatio.]
   1. The act of truncating, lopping, or cutting off.
      [1913 Webster]

   2. The state of being truncated.
      [1913 Webster]

   3. (Min.) The replacement of an edge or solid angle by a
      plane, especially when the plane is equally inclined to
      the adjoining faces.
      [1913 Webster]

WordNet (r) 3.0 (2006):

truncation
    n 1: the property of being truncated or short [syn: shortness,
         truncation]
    2: the replacement of an edge or solid angle (as in cutting a
       gemstone) by a plane (especially by a plane that is equally
       inclined to the adjacent faces)
    3: the act of cutting short; "it is an obvious truncation of the
       verse"; "they were sentenced to a truncation of their limbs"