Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

carte blanche
     n : complete freedom or authority to act