The Collaborative International Dictionary of English v.0.48:

Fiat \Fi"at\, n. [L., let it be done, 3d pers. sing., subj.
   pres., fr. fieri, used as pass. of facere to make. Cf. Be.]
   1. An authoritative command or order to do something; an
      effectual decree.
      [1913 Webster]

            His fiat laid the corner stone.       --Willis.
      [1913 Webster]

   2. (Eng. Law)
      (a) A warrant of a judge for certain processes.
      (b) An authority for certain proceedings given by the Lord
          Chancellor's signature.
          [1913 Webster]

   Fiat money, irredeemable paper currency, not resting on a
      specie basis, but deriving its purchasing power from the
      declaratory fiat of the government issuing it.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fiat money
    n 1: money that the government declares to be legal tender
         although it cannot be converted into standard specie