The Collaborative International Dictionary of English v.0.48:

Carte blanche \Carte` blanche"\ [F., fr. OF. carte paper +
   -blanc, blanche, white. See 1st Card.]
   A blank paper, with a person's signature, etc., at the
   bottom, given to another person, with permission to
   superscribe what conditions he pleases. Hence: Unconditional
   terms; unlimited authority.
   [1913 Webster]

WordNet (r) 3.0 (2006):

carte blanche
    n 1: complete freedom or authority to act