Antoninus PiusPronunciation: /ˌantəˈnʌɪnəs ˈpʌɪəs/Definition of Antoninus Pius (86-161), Roman emperor 138–61. The adopted son and successor of Hadrian, he had a generally peaceful and harmonious reign. Antoninus Pius in other Oxford dictionaries Definition of Antoninus Pius in the US English dictionary