Cassius Dīo, Coccēiānus, also known as Dio Cassius, c. AD 150–235, Roman historian from Nicaea in Bithynia. He was twice consul at Rome and governor of Africa and Dalmatia. He wrote a small book on dreams and portents, and a history of the civil wars of 193–7 (both lost), but his major work was the history of Rome (written in Greek) in eighty books, from the landing of Aeneas in Italy down to AD 229. Books 36–54, covering the years 68–10 BC, survive complete, books 55–60 (9 BC–AD 46) in an abbreviated form, and books 79–80 (AD 217–20) in part. The rest of the history has to be pieced together from the summary descriptions of Byzantine historians of the eleventh and twelfth centuries. He is an important witness for contemporary events and a valuable commentator on the political aspects of history through his own experience, but he is unreliable about republican institutions and the style is coloured by his rhetorical training.

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Roman writer who lived in the late second and early 3rd centuries ad. He refers to events in Britain from the conquest of ad 43 onwards, but his often lively narrative is not always considered entirely reliable except when he is writing about his own times.

(Cassius Dio Cocceianus) (dīo kăsh'əs), c.155–235?, Roman historian and administrator, b. Nicaea in Bithynia. He was a grandson of Dio Chrysostom. His rise in civil and military office was steady; he became a senator (c.180), praetor (193), consul (220?), proconsul in Africa (224), legate in Dalmatia (226), legate in Pannonia (227), and consul again (229). He was a good commander, but he remained in favor more for his literary works than for his abilities in office. His great work, partially extant, was a history of Rome (written in Greek) from the earliest times until Dio Cassius' own period. Of the original 80 books, 19 survive in full. They are a reputable source for the period of the later republic and the first two centuries A.D. Dio Cassius tried earnestly to study all available sources in the light of a moderate skepticism.