The Collaborative International Dictionary of English v.0.48:

Corroboration \Cor*rob`o*ra"tion\ (k?r-r?b`?-r?"sh?n), n. [Cf.
   F. corroboration.]
   1. The act of corroborating, strengthening, or confirming;
      addition of strength; confirmation; as, the corroboration
      of an argument, or of information.
      [1913 Webster]

   2. That which corroborates.
      [1913 Webster]

WordNet (r) 3.0 (2006):

corroboration
    n 1: confirmation that some fact or statement is true through
         the use of documentary evidence [syn: documentation,
         certification, corroboration]