Law is a system of rules that governs the activities and relations of people in a particular society or community. The law may consist of legislation or a set of custom and policies recognized and enforced by a legal system. It may be based on religious precepts, as with Jewish halakha or Islamic Sharia. It can also be derived through human elaboration, as with the Christian canon and English common law.

The law is enforced by police, judges and other governmental officials, but it also encompasses the social contracts, norms and values that shape a nation or region in various ways. It may be written or unwritten, and it may vary among societies in terms of its scope and complexity. It shapes politics, economics, history and culture in countless ways.

In the modern world, the law is largely the product of legislative statutes, executive orders and judicial decisions. In the United States, federal laws are codified in a series of books known as the United States Code, while state laws and regulations are contained in statutory lawbooks and court decisions. In some areas, such as aviation and railroads, a small number of national regulations preempt state law, while in others, such as family law, federal and state laws coexist.

Whether a law is written or not, and whether it is in the form of a constitution or not, it can be influenced by religion and moral philosophy. For example, a moral argument might support prohibitions against murder or theft and might justify the existence of laws prohibiting those activities. In addition, a legal philosophy such as natural law might underlie a belief in the rightness of certain types of conduct.

Many fields of law involve issues of morality, such as criminal laws, civil rights and responsibilities, or ethical business practices. Other fields focus on the specifics of a country or community: immigration law and nationality laws deal with the rights of foreigners to live in a nation, and international law deals with the interplay between nations. Family law addresses the rights of children and spouses, and commercial law relates to business and money.

The law can be a complex topic, and it is not always clear how to distinguish a legal rule from a mere suggestion or piece of advice. For example, a recommendation to eat five fruits and vegetables a day is not likely to lead to any consequences, but making obscene phone calls or stealing are both illegal actions that can have serious consequences. The concept of law is a fundamental one, and its importance extends to everything from regulating obscene phone calls to determining how much a person can own. The Oxford Reference Law Online includes concise definitions and specialized encyclopedic entries across the law, covering everything from international and corporate law to civil and criminal procedure. This extensive coverage is complemented by authoritative articles on legal philosophy and the major debates in this field. It is essential reading for researchers at every level.