1. Someone who has attained maturity or legal age: Because Susan was an adult and 21 years old, she could vole in the elections in her country.
2. The state of development of having attained full size, strength, and maturity: Organisms that con be considered to be adults are fully grown, like people, animals, and plants.
3. Etymology: from Latin adultus, past participle of adolescere "to grow up, to mature".