An adult may mean different things. In law and social norms, an adult is a human who has reached the age of majority. That age varies: in most countries, an adult is a person who is 18 years old or older. Once someone is adult, they no longer have to take directions from a parent or guardian but can make decisions for themselves, and be held responsible for their own actions. Legal penalties for crimes are different before and after the age of majority.
In contrast, a person who has not yet reached the age of majority is a minor.
There are additional laws that decide from what age a person is allowed to do something; for example, vote, drink alcohol, and have sexual intercourse. For example, in the United States, a person becomes an adult on their 18th birthday, but they cannot buy alcoholic beverages until they turn 21. To verify their age, a young person must have an I.D. card for inspection by the bartender.
There are also some cultures that says a person is an adult from an age of 13 years on, for example see Bar Mitzi, which usually corresponds to puberty.
In biology, an animal is an adult when they are sexually mature. This means that they are able to sexually reproduce.