1. In general, any standard, principle, or norm that guides conduct. Thus, any factual situation calling for a decision might be thought of as raising two questions: first, what rules apply to these facts; second, what decision should be made if those rules are to be followed?
2. In administrative law, a regulation that governs an agency's internal operations as opposed to a regulation that governs individuals and businesses in the world at large.
3. In litigation, any standard or principle by which courts resolve disputes. See, e.g., Federal Rules of Civil Procedure.