Having arrived at mature years, or attained full size and strength: as, an adult person, animal, or plant.Pertaining or relating to adults; suitable for an adult: as, adult age; an adult school.n. A person or (sometimes) an animal grown to full size and strength; one who has reached the age of manhood or womanhood.n. Specifically: In civil law, a male infant who has attained the age of fourteen, or a female infant who has attained the age of twelve.n. In common law, one of the full age of twenty-one.