Adult definition is - fully developed and mature: grown-up. How to use adult in a sentence. noun a person who is fully grown or developed or of age. a full-grown animal or plant. a person who has attained the age of maturity as specified by law.
The definition of an adult is a person who is fully grown or is 18 years of age or older, as specified by the law. A 20 year old person is an example of an adult.