Tacitus Syllabification: (Tac·i·tus)Pronunciation: /ˈtasətəs/Definition of Tacitus (circa 56-120), Roman historian; full name Publius (or Gaius) Cornelius Tacitus. His Annals (covering the years 14-68) and Histories (69-96) are major works on the history of the Roman Empire. Tacitus in other Oxford dictionaries Definition of Tacitus in the British & World English dictionary