The term adult describes any mature organism, but normally it refers to a human: one that is no longer a child and is now either a man or a woman. When someone becomes an adult, they enter adulthood.
For humans, the legal definition of entering adulthood varies between ages 17-21, depending on the region in question. Some cultures in Africa define adult at age 13, but most other civilizations consider this the teenager stage.
"Adult" also means "not considered suitable for children", in particular as euphemism for being related to sexual behaviour, e.g. adult entertainment, adult video, adult magazine, adult bookstore.