विधि या क़ानून या सन्नियम किसी नियम संहिता को कहते हैं। विधि प्रायः भलीभांति लिखी हुई दिशा व निर्देशों के रूप में होती है। समाज को सम्यक ढंग से चलाने के लिये विधि अत्यन्त आवश्यक है।

Law is a set of rules that are created and are enforceable by social or governmental institutions to regulate behavior, with its precise definition a matter of longstanding debate. It has been variously described as a science and as the art of justice. State-enforced laws can be made by a group legislature or by a single legislator, resulting in statutes; by the executive through decrees and regulations; or established by judges through precedent, usually in common law jurisdictions. Private individuals may create legally binding contracts, including arbitration agreements that adopt alternative ways of resolving disputes to standard court litigation. The creation of laws themselves may be influenced by a constitution, written or tacit, and the rights encoded therein. The law shapes politics, economics, history and society in various ways and also serves as a mediator of relations between people.