The Collaborative International Dictionary of English v.0.48:

Supersession \Su`per*ses"sion\, n. [Cf. OF. supersession. See
   Supersede.]
   The act of superseding, or the state of being superseded;
   supersedure.
   [1913 Webster]

         The general law of diminishing return from land would
         have undergone, to that extent, a temporary
         supersession.                            --J. S. Mill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

supersession
    n 1: act of replacing one person or thing by another especially
         one held to be superior [syn: supersedure,
         supersession]