Dictator was a political office of the Roman Republic.
A legal innovation of the Roman Republic, the dictator (Latin for "one who dictates (orders)") -- also known as the magister populi ("master of the peoples") -- was an extraordinary magistrate (magistratus extraordinarius) whose function was to perform extraordinary tasks exceeding the authority of any of the ordinary magistrates. The Roman Senate passed a senatus consultum authorizing the consuls to nominate a dictator, who was the sole exception to the Roman legal principles of collegiality (multiple tenants of the same office) and responsibility (being legally able to be held to answer for actions in office); there could never be more than one dictator at any one time for any reason, and no dictator could ever be held legally responsible for any action during his time in office for any reason. The dictator was the highest magistrate in degree of precedence (praetor maximus) and was attended by 24 lictors.
There were actually several different types of dictatorate. The most famous type is the dictator rei gerendae causa, who was appointed in times of military emergency for six months or for the duration of the emergency, whichever period was shorter. This dictator held absolute military and civil power in the State, and was obligated to appoint as his deputy a master of the horse (magister equitum). When the dictator left office, the office of master of the horse immediately ceased to exist. Other types of dictators were occasionally appointed for more mundane reasons: comitiorum habendorum causa (for summoning the comitia for elections), clavi figendi causa (for fixing the clavus annalis in the temple of Jupiter), feriarum constituendarum causa (for appointing holidays), ludorum faciendorum causa (for officiating at public games), quaestionibus exercendis (for holding certain trials), and legendo senatui (for filling vacancies in the Senate).
The best known dictatores rei gerendae causa were Cincinnatus and Fabius Maximus (during the Second Punic War). Thereafter this form of dictatorate fell into disuse. After the falling out of Gaius Marius and Lucius Cornelius Sulla, the latter marched on Rome and had himself appointed to an entirely new office, dictator rei publicae constituendae causa, which was functionally identical to the dictatorate rei gerendae causa except that it lacked any time limit. Sulla held this office for years before he voluntarily abdicated and retired from public life.
Julius Caesar subsequently resurrected the dictatorate rei gerendae causa in his first dictatorship, then modified it to a full year term. He was appointed dictator rei gerendae causa for a full year in 46 BC and then designated for nine consecutive one-year terms in that office thereafter, functionally becoming dictator for ten years. A year later, this pretense was discarded altogether and the Senate voted to make him dictator perpetuus (usually rendered in English as "dictator for life", but properly meaning "perpetual dictator").
After Caesar's murder on the Ides of March, his consular colleague Marcus Antonius passed a lex Antonia which abolished the dictatorate and expunged it from the constitutions of the Republic. The office was later offered to Caesar Augustus, who prudently declined it, and opted instead for tribunician power and consular imperium without holding any office other than pontifex maximus and princeps senatus -- a politic arrangement which left him as functional dictator without having to hold the controversial title or office itself.
List of Roman dictators
See also: dictator – decemviri