Definition of adult noun from the Oxford Advanced American Dictionary

        

    adult

     noun
    noun
    NAmE//əˈdʌlt//
     
    , NAmE//ˈædʌlt//
     
     
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  1. 1a fully grown person who is legally responsible for their actions synonym grown-up2 Children must be accompanied by an adult. Why can't you two act like civilized adults?
  2. 2a fully grown animal The fish return to the river as adults in order to breed.
See the Oxford Advanced Learner's Dictionary entry: adult