Titus Aelius Hadrianus Antoninus Pius ( 19 September 86 – 7 March 161) was Roman emperor from AD 138 to 161. He was the fourth of the Five Good Emperors from the Nerva–Antonine dynasty.
Born into a senatorial family, Antoninus held various offices during the reign of Emperor Hadrian. He married Hadrian's niece Faustina, and Hadrian adopted him as his son and successor shortly before his death. Antoninus acquired the cognomen Pius after his accession to the throne, either because he compelled the Senate to deify his adoptive father, or because he had saved senators sentenced to death by Hadrian in his later years. His reign is notable for the peaceful state of the Empire, with no major revolts or military incursions during this time. A successful military campaign in southern Scotland early in his reign resulted in the construction of the Antonine Wall.
 
Two days before his death, the biographer reports, Antoninus was at his ancestral estate at Lorium, in Etruria.  about twelve miles (19 km) from Rome.[ He ate Alpine cheese at dinner quite greedily. In the night he vomited; he had a fever the next day. The day after that, he summoned the imperial council, and passed the state and his daughter to Marcus. The emperor gave the keynote to his life in the last word that he uttered: when the tribune of the night-watch came to ask the password, he responded, "aequanimitas" (equanimity)  He then turned over, as if going to sleep, and died. His death closed out the longest reign since Augustus (surpassing Tiberius by a couple of months). His record for the second-longest reign would be unbeaten for 168 years, until 329 when it was surpassed by Constantine the Great.