Rule
1. An established standard, guide or regulation; a principle or regulation set up by authority, prescribing or directing action or forbearance; as, the rules of a legislative body, of a company, court, public office, of the law, of ethics.
2. A regulation made by a court of justice or public office with reference to the conduct of business therin.
3. An order made by a court, at the instance of one of the parties to a suit, commanding a ministerial officer, or the opposite party, to do some act, or to show cause why some act should not be done. It it usually upon some interlocutory matter, and has not the force or solemnity of a decree or judgment.
4. "Rule" sometimes means a rule of law. Thus, we speak of the rule against perpetuties; the rule in Shelley's Case, etc.
From Black's Law Dictionary first edition, 1891, http://books.google.com/books?id=PNQgVoT2_aQC