The Collaborative International Dictionary of English v.0.48:

Asseveration \As*sev`er*a"tion\, n. [L. asseveratio.]
   The act of asseverating, or that which is asseverated;
   positive affirmation or assertion; solemn declaration.
   [1913 Webster]

         Another abuse of the tongue I might add, -- vehement
         asseverations upon slight and trivial occasions. --Ray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

asseveration
    n 1: a declaration that is made emphatically (as if no
         supporting evidence were necessary) [syn: assertion,
         averment, asseveration]