The term adult has three distinct meanings. It indicates a grown man or woman. It may also mean a plant or animal that has reached full growth, or one who is legally of age; as opposed to a minor.

Adulthood can be defined in terms of biology, psychological adult development, law, personal character, or social status. These different aspects of adulthood are often inconsistent and contradictory. A person may be biologically an adult, and have adult behavioral characteristics but still be treated as a child if they are under the legal age of majority. Conversely one may legally be an adult but possess none of the maturity and responsibility that define adult character.

Coming of age is the event; passing a series of tests to demonstrate the child is prepared for adulthood; or reaching a specified age, sometimes in conjunction with demonstrating preparation. Most modern societies determine legal adulthood based on reaching a legally-specified age without requiring a demonstration of physical maturity or preparation for adulthood.

Adult, especially in the sense of entertainment or other diversion, frequently appears as a euphemism for being related to sexual behaviour. Adult toys and adult games—which terms refer to games and toys that are closely related to sex, do not generally refer only to games or toys with which children are simply incompatible—are in this category, for example. This usage does indicate unsuitability for children, but the more immediate meaning is closer to "not legal for children." Adult education, however, does simply mean education for adults, especially for those past the usual age for either high school or university.

Some propose that moving into adulthood involves an emotional structuring of denial. This process becomes necessary to cope with one's own behavior, especially in uncomfortable situations, and also the behavior of others.

Biological adulthood

Adulthood is generally understood as the time when physical maturation is complete. One reaches their maximum height and secondary sex characteristics form such as body hair and facial hair, voice lowers in pitch (especially noticeable in men), and menses begin (women). Natural sleep patterns change in adulthood, as adults typically require less sleep than during adolescence. One aspect people do not specify is what (psychologically) an adult is. A common theory is that adulthood is the real test of life, to experience the world from a first-person standpoint instead of through the parents. Then the adult can pass those experiences down to younger people and they can experience them when they become adults.

Personal characteristics

There are some qualities that symbolize adultness in most cultures. There is not always a correlation between the qualities and the physical age of the person.