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)

Other universes
In Ecto-Space, the Great Kingdom of the Seven Systems, based on the planet Jokastarna, was ruled by an Emperor or Empress, whose consort would also gain either title, as dictated by gender. Known Emperors included Trist and Lukor. (AUDIO: The Choice)