Formal and official:
* Decree: A formal order or proclamation issued by a sovereign or authority.
* Proclamation: A public and official announcement.
* Command: A firm order or instruction.
* Mandate: An official order or authorization.
* Regulation: A rule or principle that governs conduct or procedure.
* Statute: A written law.
Less formal:
* Order: A command or instruction.
* Rule: A principle or guideline that governs behavior.
* Directive: A guideline or instruction.
* Pronouncement: A formal or authoritative statement.
* Declaration: A formal statement announcing a position or intention.
The best synonym will depend on the specific context of the edict you are referring to.