An adult is a person who has reached full physical maturity, typically around age 18 in most countries. When you become an adult, you gain new rights and responsibilities: you can vote, sign contracts, and make your own medical decisions. Becoming an adult also involves developing qualities like taking responsibility for your actions, keeping your commitments, and thinking about how your choices affect others.

Sometimes people use adult to describe behavior or attitudes rather than just age. When someone handles a disappointment calmly instead of throwing a tantrum, others might say they're being very adult about it. When a student admits a mistake and fixes it without making excuses, that's acting like an adult.

The opposite of adult is child or juvenile. In the animal kingdom, adults are fully grown members of a species, like an adult eagle that can hunt and survive on its own, versus a chick that still needs parental care.

People don't automatically become wise and responsible just by getting older. Some adults handle challenges with grace and good judgment, while others struggle. Maturity develops through experience, learning from mistakes, and making thoughtful choices.