In law, a minor is someone under a certain age, usually the age of majority, which demarcates an underage individual from legal adulthood. The age of majority depends upon jurisdiction and application, but it is commonly 18. Minor may also be used in contexts that are unconnected to the overall age of majority. For example, the smoking and drinking age in the United States is 21, and younger people below this age are sometimes called minors in the context of tobacco and alcohol law, even if they are at least 18. The terms underage or minor often refer to those under the age of majority, but may also refer to a person under other legal age limits, such as the age of consent, marriageable age, driving age, voting age, etc. Such age limits are often different from the age of majority.

The concept of minor is not sharply defined in most jurisdictions. The age of criminal responsibility and consent, the age at which school attendance is no longer compulsory, the age at which legally binding contracts can be entered into, and so on may be different from one another.

In many countries, including Australia, Serbia, India, Brazil, Croatia, Colombia, and the UK a minor is defined as a person under the age of 18. In the United States, where the age of majority is set by individual states, "minor" usually refers to someone under 18 but can in some areas (such as alcohol, gambling, and handguns) mean under 21. In the criminal justice system a minor may be tried and punished either "as a juvenile" or "as an adult".

In Taiwan and Thailand, a minor is a person under 20 years of age, and, in South Korea, a person under 19 years of age. In New Zealand law, the age of majority is 20 years of age as well, but most of the rights of adulthood are assumed at lower ages: for example, entering contracts and having a will are allowed at 15, while the drinking and voting age are both at 18.