General terms:
* Decree: A formal and authoritative order or proclamation, especially issued by a sovereign or other high authority.
* Edict: A formal and authoritative proclamation or order, often issued by a ruler or government.
* Mandate: An official order or commission to do something.
* Directive: An official instruction or order, often issued by a government or organization.
* Command: An order given by someone in authority.
More specific terms:
* Writ: A formal written order issued by a court, often commanding someone to do something or appear in court.
* Summons: A notice to appear in court.
* Subpoena: A legal writ ordering a person to appear in court to give evidence.
* Injunction: A court order that prohibits someone from doing something.
* Regulation: A rule or order issued by a government or other authority.
Informal terms:
* Instruction: A direction or order given to someone.
* Request: A polite or formal demand for something.
The best word to use will depend on the specific situation.