Emperor

An emperor was a male monarch, usually the sovereign ruler of an empire or another type of imperial realm.

An empress, the female equivalent, might be an emperor's wife (empress consort) or, in Queen Victoria's case, a woman who ruled in her own right (empress regnant). (TV: Tooth and Claw) Emperors were generally recognised to be of a higher honour and rank than kings.

Emperors and their domains
The Dalek Emperor ruled the Dalek Empire. There were at least six different Dalek Emperors, notably including Davros, (TV: Remembrance of the Daleks) the Golden Emperor, (COMIC: Invasion of the Daleks) and the Emperor in 200100. (TV: The Parting of the Ways)

There was a Cyber-Emperor, who ruled over the Cyber-Empire, (COMIC: Kane's Story, Frobisher's Story) and a Uvodni Emperor who ruled the Uvodni Empire. (TV: Warriors of Kudlak)

In the post-War universe, the universe was ruled by the Emperor, one of the surviving elementals, from the Needle. (PROSE: Father Time)

On Earth, the leader of the Roman Empire was called Caesar. (TV: The Romans, et. al.) In the Empire of Empires, the Caesar of Roma I was given the title Prime Emperor. (PROSE: Warlords of Utopia)

Charlemagne reigned over the Holy Roman Empire and Franz Joseph was emperor over Austria.