<p><strong>Noun</strong></p><ul><li>A person who is fully grown or developed or of age.</li><li>A full-grown animal or plant.</li><li>A person who has attained the age of maturity as specified by law.</li><li>Usually the old. (used with a plural verb) Old persons collectively: appropriations to care for the old.</li><li>A person or animal of a specified age or age group (used in combination): a horse race for three-year-olds.</li><li>Old or former time, often time long past: days of old.</li></ul><p><strong>Adjective</strong></p><ul><li>Having attained full size and strength; grown up; mature: an adult person, animal, or plant.</li><li>Of, relating to, or befitting adults.</li><li>Intended for adults; not suitable for children: adult entertainment.</li></ul><p><strong>Verb</strong></p><ul><li>(Used without object) Informal. (Of a young person) To do things and assume responsibilities that are associated with being an adult; act like an adult (usually used facetiously about minor accomplishments): I cook dinner for myself most nights—guess that means I'm adulting.</li></ul>