Mar 06, · 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. 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 night. If you're still a child, then you are not an adult — that's the noun version.
1. grown-up, mature person, person of mature age, grown or grown-up person (man or woman) Children under 14 must be accompanied by an adult. Adult definition is - fully developed and mature: grown-up. How to use adult in a sentence.