Definify.com
Webster 1913 Edition
Fiat
Fi′at
,Noun.
 [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. 
His 
fiat 
laid the corner stone. Willis.
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. 
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.