Caesar (ˈkae̯.sar English () Caesars; Latin () Caesares; in Greek: Καῖσαρ Kaîsar) is a title of imperial character. It derives from the cognomen of Julius Caesar, a Roman dictator. The change from being a familial name to a title adopted by the Roman emperors can be traced to AD 68, following the fall of the Julio-Claudian dynasty.
The first known individual to bear the cognomen of "Caesar" was Sextus Julius Caesar, who is likewise believed to be the common ancestor of all subsequent Julii Caesares. Sextus' great-grandson was the dictator Gaius Julius Caesar. After he seized control of the Roman Republic following his war against the Senate, he adopted the title of dictator perpetuo ("dictator in perpetuity"), a title he only held for about a month before he was assassinated in 44 BC. Julius Caesar's death did not lead to the restoration of the Republic, and instead led to the rise of the Second Triumvirate, which was made up of three generals, including Julius' adopted son Gaius Octavius.
Following Roman naming conventions, Octavius adopted the name of his adoptive father, thus also becoming "Gaius Julius Caesar", though he was often called "Octavianus" to avoid confusion. He usually styled himself simply as "Gaius Caesar" to emphasize his relationship with Julius Caesar. Eventually, distrust and jealousy between the triumvirs led to a lengthy civil war which ultimately ended with Octavius gaining control of the entire Roman world in 30 BC. In 27 BC, Octavius was given the honorific Augustus by the Senate, adopting the name of "Imperator Caesar Augustus". He had previously dropped all his names except for "Caesar", which he treated as a nomen, and had adopted the victory title imperator ("commander") as a new praenomen.
As a matter of course, Augustus' own adopted son and successor, Tiberius, followed his (step)father's example and bore the name "Caesar" following his adoption on 26 June 4 AD, restyling himself as "Tiberius Julius Caesar". Upon his own ascension to the throne, he styled himself as "Tiberius Caesar Augustus".