Originally the name of a Roman aristocratic family and assumed by Augustus as the adopted son of the dictator C. Julius Caesar, and passed on by Augustus to his adopted son Tiberius. Then the title continued to be used by Caligula, Claudius, and Nero. After that, succeeding emperors retained the name not only for themselves but also for the heir presumptive to the throne. Their power was exercised in the empire by a remarkable combination of legal authority and political acumen. The names of the ruling Caesars in the NT era are:

27 BCE–14 CE Augustus
14–37 Tiberius
37–41 Gaius (given the surname Caligula by his troops)
41–54 Claudius
54–68 Nero
68–9 Galba
69 Otho; Vitellius
69–79 Vespasian
79–81 Titus
81–97 Domitian
98–117 Trajan
117–138 Hadrian