Publius (or Gaius) Cornelius Tacitus (; Classical Latin: [ˈtakɪtʊs]; c.  56 – c.  120 AD) was a senator and a historian of the Roman Empire. Tacitus is considered to be one of the greatest Roman historians. He lived in what has been called the Silver Age of Latin literature, and is known for the brevity and compactness of his Latin prose, as well as for his penetrating insights into the psychology of power politics.

The surviving portions of his two major works—the Annals and the Histories—examine the reigns of the emperors Tiberius, Claudius, Nero, and those who reigned in the Year of the Four Emperors (69 AD). These two works span the history of the Roman Empire from the death of Augustus, in 14 AD, to the years of the First Jewish–Roman War, in 70 AD. There are substantial lacunae in the surviving texts, including a gap in the Annals that is four books long.

Tacitus' other writings discuss oratory (in dialogue format, see Dialogus de oratoribus), Germania (in De origine et situ Germanorum), and the life of his father-in-law, Agricola, the general responsible for much of the Roman conquest of Britain, mainly focusing on his campaign in Britannia (De vita et moribus Iulii Agricolae).