Many words exist that refer to an expectation expressed by an authoritative person or entity, and sometimes it’s difficult to distinguish such terms when one tries to convey a reference to a law, an order, or a rule. Here are some synonyms for those words, along with definitions.
1. Behest: an order or prompting
2. Charge: an order, or information given to a jury about the applicability of law to a specific case
3. Command: an order, especially one given by a superior officer in a military or law enforcement hierarchy, or a signal that leads to an action
4. Commandment: something ordered
5. Decree: an order, or a decision by a judicial or religious official
6. Demand: an order with an expectation of compliance and a consequence for noncompliance
7. Dictate: an order or rule; also synonymous with injunction and prescription
8. Direction: an order, or an indication of how actions should occur in a performance or how music should be played
9. Directive: an order issued by a government entity, an official, or an organization
10. Edict: an order or proclamation
11. Imperative: an order or a rule, or a similar statement
12. Injunction: an order, or a legal requirement to do or not do something
13. Instruction: an order or direction; also synonymous with precept
14. Mandate: an order handed down from one court or other judicial body to another, or an authorization granted to a government official or a government, an order for establishment of a government, or a territory under such an order
15. Ordinance: a law or order established by a government entity, especially on the municipal level, a specified ceremony or custom, or something ordered by a god or determined by fate
16. Precept: an order given to an official by the person’s superior, or an order or a principle that is to be considered a general rule
17. Prescription: a right or title, an establishment of a rule, a claim or rule or tradition based on custom, a direction for preparation and/or use of a medicine or similar treatment, or the medicine or treatment itself
18. Regulation: a procedural rule, or an order enforceable by a government agency, or the act or state of developing one or more such rules
19. Statute: a law developed by a government’s legislative branch, the documentation for creating an agency and specifying its authority and scope, or a rule established by a corporation
20. Word: a direction or order
This list will be good to keep in mind for writing fantasy/sci-fi stories.