Law is a system of rules created and enforced by social or governmental institutions. The precise definition of “law” is a matter of debate, but it usually refers to strong rules made by an authority that must be obeyed or punished. For example, most countries have laws against stealing and they punish people who break those laws. A legal system may be established by a group legislature, resulting in statutes; by the executive through decrees or regulations; or by judges through precedent, as in common law systems. Private individuals can also create legally binding contracts.

Some commentators on law emphasize that the law is not merely a set of rules handed down from a sovereign, but also reflects a moral position. For instance, the prohibition against insider trading might be viewed as a reflection of the concept of fairness, a moral concern. Likewise, the principle of due process (fundamental fairness and decency in government actions) might be seen as an ethical position against cruelty.

There are many branches of law, including criminal law (which deals with crimes such as murder or robbery), civil law (such as contract law or tort law) and administrative law (such as immigration law). Other areas include family law, which encompasses divorce proceedings and rights to children; aviation law, which covers all the regulations concerning flying a plane; and biolaw, which intersects with the biological sciences. See also constitution; constitutionalism; law, philosophy of; law and society; and legal history.