Emperor

An emperor was a male monarch, usually the sovereign ruler of an empire or another type of imperial realm. (TV: Nightmare in Silver)

An empress, the female equivalent, could 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) An empress' husband was sometimes an emperor consort. (AUDIO: Empire of the Racnoss) Emperors were generally recognised to be of a higher honour and rank than kings.

Emperors and their domains
Omegon claimed that the early Time Lords had briefly made him their Emperor after he granted them time travel. (PROSE: K9 and the Time Trap)

One Racnoss Empress had one emperor consort before she chose a new emperor. (AUDIO: Empire of the Racnoss)

A Dalek Emperor usually ruled the Dalek Empire. Though there were brief periods in Dalek history when two different Emperors coexisted peacefully in different areas of the universe, (PROSE: The Dalek Problem) it was usually singular. The first Dalek ever spent most of his life as Emperor, inhabiting a variety of casings; (COMIC: Invasion of the Daleks, PROSE: War of the Daleks, TV: The Evil of the Daleks) after a period during which Davros took over as Emperor of his own faction of Imperial Daleks, (TV: Remembrance of the Daleks) the Dalek Prime was later consciously imitated by the Emperor of the Restoration, (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe, The Restoration Empire) A A Dalek Emperor distinct from Davros headed the Empire throughout the Last Great Time War, (TV: The Parting of the Ways) although his identity was disputed. (PROSE: Dalek Combat Training Manual, AUDIO: Restoration of the Daleks, etc.) The New Dalek Empire and the New Dalek Paradigm established after the Time War both had a Supreme Dalek as an ultimate leader at their inceptions, (TV: The Time of the Doctor, The Stolen Earth) but the Paradigm later experimented with bringing back an Emperor position. (GAME: The Eternity Clock, City of the Daleks)

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, COMIC: Miranda) He was a potential future self of the Doctor. (PROSE: The Gallifrey Chronicles)

On Earth, the leader of the Roman Empire was called Caesar, beginning with Julius Caesar. Nero, (TV: The Romans) Augustus, Tiberius, (AUDIO: 100 BC) Claudius, (PROSE: Of the City of the Saved...) Hadrian, (AUDIO: Many Happy Returns) and Valentinian were Caesars. (PROSE: Combat Magicks) In the Empire of Empires, the Caesar of Roma I was given the title Prime Emperor. (PROSE: Warlords of Utopia)

Justinian was Emperor of Byzantium in 540. (PROSE: The Secret History)

Kublai Khan was Emperor of the Mongols and China. (TV: Marco Polo)

Ludens Nimrod Kendrick Cord Longstaff XLI was the Emperor and Defender of Humanity and Imperator of Known Space. (TV: Nightmare in Silver)

The nation of Japan also had its own Japanese emperor while the prominent power in Asia during the first half of the 20th century. (PROSE: The Shadow of Weng-Chiang)

Charlemagne reigned over the Holy Roman Empire (PROSE: The Lonely Computer) and Franz Joseph was emperor over Austria. (AUDIO: The Silver Turk) Napoléon Bonaparte was the Emperor of France. His nephew, Napoleon III, was emperor in the 1860s. (PROSE: World Game)

Ludens Nimrod Kendrick Cord Longstaff XLI was a human emperor, known as the imperator of space. (TV: Nightmare in Silver)

In a timeline where River Song caused time to collapse when she refused to kill the Doctor, Winston Churchill was the Holy Roman Emperor in Britain. (TV: The Wedding of River Song)