An adult is a mature, fully developed person. An adult has reached the age when they are legally responsible for their actions.

An adult is a fully developed animal.

Adult means relating to the time when you are an adult, or typical of adult people.

If you say that someone is adult about something, you think that they act in a mature, intelligent way, especially when faced with a difficult situation.

You can describe things such as films or books as adult when they deal with sex in a very clear and open way.