The Collaborative International Dictionary of English v.0.48:

Avouchment \A*vouch"ment\, n.
   The act of avouching; positive declaration. [Obs.] --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

avouchment
    n 1: a statement asserting the existence or the truth of
         something [syn: avowal, avouchment, affirmation]