In contrast to a non-adult or "minor", a legal adult is a person who has attained the age of majority and is therefore regarded as independent, self-sufficient, and responsible. 3 days ago · The meaning of ADULT is fully developed and mature : grown-up. How to use adult in a sentence. An adult under English law is someone over 18 years old. Adults pay an admission charge but children get in free. Adult is noun and an adjective, both of which refer to something mature, no longer a child. A full-grown person is called an adult: they can vote, rent cars, and choose to eat sugary cereal for dinner every … 1. Fully developed and mature. Relating to, intended for, or befitting adults: adult education. Containing or dealing in explicitly sexual material; pornographic: adult movies. ADULT definition: a person who is fully grown or developed or of age. See examples of adult used in a sentence. An adult under English law is someone over 18 years old. Adults pay an admission charge but children get in free. Find your favorite shows, watch free 24/7 marathons, get event info, or just stare blankly.