Formal:
* Statute: A written law passed by a legislative body.
* Act: A law enacted by a legislative body.
* Ordinance: A law enacted by a local government.
* Enactment: The process of making a law.
* Code: A collection of laws organized by subject matter.
Informal:
* Law: A general term for a rule that is enforced by the state.
* Regulation: A rule or directive issued by a government agency.
* Policy: A guiding principle that influences the actions of a government or organization.
Other:
* Bill: A proposed law that has not yet been passed.
* Resolution: A formal statement of opinion or intent.
The best word to use will depend on the specific context and the level of formality you desire.