Adult is a word that can have several meanings:
- In the sciences of biology or psychology it can indicate someone who has attained physical or psychological maturity. Antonym for this is child (though child is also used in law synonymously with minor so this is a source of confusion) or intermediary stages described with nouns/adjectives like pubescent or adolescent
- In society and law, someone who has reached the age of majority and can legally vote to influence their nation. In most instances this indicates people who have passed their 18th birthday, antonym of this use is minor
- In slang, various things people do are considered a passage into this, like earning a paycheck, driving a car, living on your own, getting married, etc.