A more thorough explanation:

Definition: An order or decree, especially one that is arbitrary or comes from a court.

Examples:

• The king issued a fiat that all citizens must pay a new tax.
• The judge issued a fiat that the trial would begin on a certain date.

These examples show how a person in power can issue a fiat, which is a command that must be followed. It can come from a king or a judge, and it can be about anything from taxes to court dates.