The Collaborative International Dictionary of English v.0.48:

Demission \De*mis"sion\, n. [L. demissio, fr. demittere. See
   Demit.]
   1. The act of demitting, or the state of being demitted; a
      letting down; a lowering; dejection. "Demission of mind."
      --Hammond.
      [1913 Webster]

            Demission of sovereign authority.     --L'Estrange.
      [1913 Webster]

   2. Resignation of an office. [Scot.]
      [1913 Webster]