The Collaborative International Dictionary of English v.0.48:

Sponsion \Spon"sion\ (-sh[u^]n), n. [L. sponsio, fr. spondere,
   sponsum, to promise solemnly.]
   1. The act of becoming surety for another.
      [1913 Webster]

   2. (Internat. Law) An act or engagement on behalf of a state,
      by an agent not specially authorized for the purpose, or
      by one who exceeds the limits of authority.
      [1913 Webster]