In law, common law (also known as judicial precedent or judge-made law, or case law) is the body of law created by judges and similar quasi-judicial tribunals by virtue of being stated in written opinions. The defining characteristic of “common law” is that it arises as precedent. In cases where the parties disagree on what the law is, a common law court looks to past precedential decisions of relevant courts, and synthesizes the principles of those past cases as applicable to the current facts. If a similar dispute has been resolved in the past, the court is usually bound to follow the reasoning used in the prior decision (a principle known as stare decisis). If, however, the court finds that the current dispute is fundamentally distinct from all previous cases (called a "matter of first impression"), and legislative statutes are either silent or ambiguous on the question, judges have the authority and duty to resolve the issue (one party or the other has to win, and on disagreements of law, judges make that decision). The court states an opinion that gives reasons for the decision, and those reasons agglomerate with past decisions as precedent to bind future judges and litigants. Common law, as the body of law made by judges, stands in contrast to and on equal footing with statutes which are adopted through the legislative process, and regulations which are promulgated by the executive branch. Stare decisis, the principle that cases should be decided according to consistent principled rules so that similar facts will yield similar results, lies at the heart of all common law systems.
The common law—so named because it was "common" to all the king's courts across England—originated in the practices of the courts of the English kings in the centuries following the Norman Conquest in 1066. The British Empire later spread the English legal system to its far flung colonies, many of which retain the common law system today. These "common law systems" are legal systems that give great weight to judicial precedent, and to the style of reasoning inherited from the English legal system.
Today, one-third of the world's population lives in common law jurisdictions or in systems mixed with civil law, including Antigua and Barbuda, Australia, Bahamas, Bangladesh, Barbados, Belize, Botswana, Burma, Cameroon, Canada (both the federal system and all its provinces except Quebec), Cyprus, Dominica, Fiji, Ghana, Grenada, Guyana, Hong Kong, India, Ireland, Israel, Jamaica, Kenya, Liberia, Malaysia, Malta, Marshall Islands, Micronesia, Namibia, Nauru, New Zealand, Nigeria, Pakistan, Palau, Papua New Guinea, Philippines, Sierra Leone, Singapore, South Africa, Sri Lanka, Trinidad and Tobago, the United Kingdom (including its overseas territories such as Gibraltar), the United States (both the federal system and 49 of its 50 states), and Zimbabwe. Some of these countries have variants on common law systems. In these countries, common law is considered synonymous with case law.