Adult (noun)

1. A person who has reached the age of legal majority and is therefore regarded as fully responsible for their actions.
2. An animal that has reached maturity.

Origin:

The word 'adult' comes from the latin word 'adultus' which means 'fully grown' or 'matured'.

Examples:

1. She became an adult at the age of 18.
2. Adults are expected to act responsibly and make mature decisions.
3. The adult elephant weighed over 10,000 pounds.
4. Adults are not allowed to enter the amusement park unless they are accompanied by a child.
5. The adult lion was the leader of the pride.