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(covers information from several alternate timelines)

Emperor or empress was a title given to a ruler of an empire or any other imperial state. An empress referred to a woman who either ruled an empire or was the head of state of an empire. This term could also refer to a woman who was the consort of a reigning emperor. The title "emperor" could occasionally refer to a female ruler of the title, at that ruler's discretion.

The Romulan Star Empire was ruled by an emperor during a point in its past, but political power rested with the Romulan Senate and the praetor. Q, however, once told Captain Kathryn Janeway that he could have mated with the Romulan Empress for the purpose of creating a child, but that he instead chose his favorite Human, "Kathy." (VOY: "The Q and the Grey")

Despite Q's reference to the existence of a Romulan empress in 2373, political power within the Romulan Empire has always been depicted to lie with the Praetor and the Senate. Since Q had the ability to travel through time; his options for a potential mate might have included a past Romulan Empress.
The novel Probe, and the short story Blood Sacrifice have suggested the Romulan Emperor to be a ceremonial position, akin to the Emperor of Japan.

Similarly, the Klingons were ruled by an emperor until the mid-21st century, when power passed to the chancellor and the High Council. In 2369, a clone of Kahless the Unforgettable assumed the title of emperor, with merely symbolic power. (TNG: "Rightful Heir"; DS9: "The Way of the Warrior", "The Sword of Kahless")

In the mirror universe, emperors ruled the Terran Empire throughout the 22nd and 23rd century. In 2155, Hoshi Sato declared herself empress of the Terran Empire after seizing control of the USS Defiant and ordering Starfleet's unconditional surrender. (ENT: "In a Mirror, Darkly, Part II") Philippa Georgiou had become the Terran Empire's Emperor by 2256. (DIS: "Despite Yourself", "The Wolf Inside") Marlena Moreau suggested that Captain Kirk might aspire to become emperor. (TOS: "Mirror, Mirror")

On the planet 892-IV, the Roman Empire was by 2268 ruled by emperors who could trace their lines back two thousand years to their own Julius and Augustus Caesars. (TOS: "Bread and Circuses")

After negotiating Cardassia's annexation into the Dominion in 2373, Dukat effectively became the emperor of the Cardassian people, though still a servant of the Founders. However, he chose to maintain his rank of gul as opposed to legate because he felt it was more "hands on", while emperor, president, first minister, and emissary seemed too "pretentious." (DS9: "Ties of Blood and Water")

List of emperors and empresses


Terran Empire

Klingon Empire


Although not stated, the Alsuran, Andorian, Annari, Kelvan, Motali, Slaver, and Tkon Empires likely had an emperor at one point.