What word means a rule or regulation?

The word that means a rule or regulation, on the USCIS reading vocabulary list, is Law. A law is a rule of conduct made and enforced by a sovereign authority, and the U.S. legal system has several kinds of law arranged in a hierarchy.

The Constitution of the United States is the supreme law of the land, as Article VI's Supremacy Clause declares: "This Constitution, and the Laws of the United States which shall be made in Pursuance thereof; and all Treaties made... under the Authority of the United States, shall be the supreme Law of the Land." Below the Constitution sit federal statutes (laws enacted by Congress and signed by the President or passed over a veto), federal treaties (also part of the supreme law of the land), federal regulations (issued by executive agencies under authority delegated by statute), state constitutions, state statutes, state regulations, and local ordinances.

Federal law generally prevails where it conflicts with state law on a matter within Congress's enumerated powers. Common law, developed through court decisions, fills gaps in many areas not covered by statute, particularly in tort, contract, and property law at the state level.

The rule-of-law principle holds that everyone (including government officials, the President, and judges) must obey the law, and the principle is reinforced by the requirement that all federal and state officials take an oath to support the Constitution and by judicial review. Citizens are expected to obey federal, state, and local laws.

The Latin phrase lex (law) is the root of words such as legal, legislator, and legitimate. On the reading test Law may appear in a sentence such as "What is the supreme law of the land?" or "Citizens must obey the law."