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
    n 1: a legally binding command or decision entered on the court
         record (as if issued by a court or judge); "a friend in New
         Mexico said that the order caused no trouble out there"
         [syn: decree, edict, fiat, order, rescript]

Moby Thesaurus II by Grady Ward, 1.0:

40 Moby Thesaurus words for "fiat":
   appointment, authority, authorization, brevet, bull, certification,
   clearance, countenance, declaration, decree, decree-law,
   decreement, decretal, decretum, dictum, diktat, edict, edictum,
   empowerment, enabling, endorsement, enfranchisement, entitlement,
   ipse dixit, law, ordinance, ordonnance, proclamation,
   pronouncement, pronunciamento, ratification, rescript, rule,
   ruling, sanction, senatus consult, senatus consultum, ukase,
   warrant, warranty



Bouvier's Law Dictionary, Revised 6th Ed (1856):

FIAT, practice. An order of a judge, or of an officer, whose authority, to
be signified by his signature, is necessary to authenticate the particular
acts.