The definition of a fiat is an order or decree, or any arbitrary order.
(noun)An example of a fiat is a legally binding decree that is issued by a king.
See fiat in Webster's New World College Dictionary
noun
Origin: L, 3d pers. sing., pres. subj., of fieri, to become, come into existence < IE base *bheu- > be
See fiat in American Heritage Dictionary 4
noun
Origin:
Origin: Medieval Latin
Origin: , from Latin, let it be done
Origin: , third person sing. present subjunctive of fierī, to become, to be done; see bheuə- in Indo-European roots
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