The Collaborative International Dictionary of English v.0.48:

Insertion \In*ser"tion\, n. [L. insertio: cf. F. insertion. See
   Insert.]
   [1913 Webster]
   1. The act of inserting; as, the insertion of scions in
      stocks; the insertion of words or passages in writings.
      [1913 Webster]

   2. The condition or mode of being inserted or attached; as,
      the insertion of stamens in a calyx.
      [1913 Webster]

   3. That which is set in or inserted, such as a word or
      passage in a composition, or a narrow strip of embroidered
      lace, muslin, or cambric; as, there were numerous
      insertions and corrections to the first draft.
      [1913 Webster +PJC]

   4. (Anat.) The point or part by which a muscle or tendon is
      attached to the part to be moved; -- in contradistinction
      to its origin.
      [1913 Webster]

   Epigynous insertion (Bot.), the insertion of stamens upon
      the ovary.

   Hypogynous insertion (Bot.), insertion beneath the ovary.
      [1913 Webster]

WordNet (r) 3.0 (2006):

insertion
    n 1: a message (spoken or written) that is introduced or
         inserted; "with the help of his friend's interpolations his
         story was eventually told"; "with many insertions in the
         margins" [syn: interpolation, insertion]
    2: the act of putting one thing into another [syn: insertion,
       introduction, intromission]