A command to do something.
- Nuttall's Standard dictionary of the English language. By Nuttall, P.Austin. Published 1914. (noun) (n.) (noun)
A command that something be done; an order of a court authorizing certain proceedings, as in bankruptcy.
- The Winston Simplified Dictionary. By William Dodge Lewis, Edgar Arthur Singer. Published 1919. (noun) (n.) (noun)
A formal or solemn command; a decree; an order.
- Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874. (noun) (n.) (noun)
A formal or solemn command; a decree.
- The american dictionary of the english language. By Daniel Lyons. Published 1899. (noun) (n.) (noun)
A command; decree.
- The Clarendon dictionary. By William Hand Browne, Samuel Stehman Haldeman. Published 1894. (noun) (n.) (noun)

Usage examples:

But the fiat must come from him.
- Mary Wollstonecraft by Elizabeth Robins Pennell
The fiat delivered in that ice- ax voice of his, cut through the polyglot murmur of the visitors gathered in the shining bare salon, all mirrors and decorations of artificial iris.
- The Disturbing Charm by Berta Ruck
I leave to mathematicians the task of calculating what are the chances of such an institution springing up in so great a multitude of nations according to an identical rule, guarded in all of them with whatever protection religion and law could afford, except by the fiat of a Creator in the manner described by Moses.
- Church and State as Seen in the Formation of Christendom by T. W. Allies