The term adult has at least three distinct meanings. It can indicate a biologically grown or mature person. It may also mean a plant, animal, or person who has reached full growth or alternatively is capable of reproduction, or a person who has attained the legally fixed age of majority; as opposed to a minor.

Adulthood can be defined in 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 behavior 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 an 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.

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

Biological adulthood

Historically and cross-culturally, adulthood has been determined primarily by the start of puberty (the appearance of secondary sex characteristics such as menstruation in women, ejaculation in men, and pubic hair in both sexes). In the past a person usually moved from the status of child directly to the status of adult, often with this shift being marked by some type of coming-of-age test or ceremony.

Today as in the past, most medical and general English dictionaries define childhood as the period from infancy to puberty, thus historically adulthood began with puberty. After the social construct of adolescence was created, adulthood split into two forms: social adulthood and biological adulthood. Thus, there are now two primary forms of adults: biological adults (people who have attained reproductive ability, are fertile, or who evidence secondary sex characteristics) and social adults (people who are recognized by their culture and/or law as being adults). Depending on the context, adult can indicate either definition.

Although few or no established dictionaries provide a definition for the two word term biological adult, the first definition of adult in multiple dictionaries includes “the stage of the life cycle of an animal after reproductive capacity has been attained”. Thus, the base definition of the word adult is the period beginning at puberty. Although this is the primary definition of the base word adult, the two word term biological adult stresses or clarifies that the original definition, based on the beginning of puberty, is being used (that is, the organism has matured to the biologically important point of being able to reproduce).