Multiple realities
(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.

Many civilizations, including the Klingons, Romulans, and Andorians, have used imperial titles at various points in their history; however, few appear to have been ruled by emperors during the period when they came into contact with Humans. The Romulan Star Empire was ruled by an emperor during a point in its past, but political power rests 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 shown to lie with the Praetor and the Senate (VOY: "The Q and the Grey"). Notably, Q has the ability to travel through time; his options for a potential mate might have included a past Romulan Empress.
The non-canon novel Probe, and the short story Blood Sacrifice have suggested the Romulan Emperor is a ceremonial position, akin to the Emperor of Japan.

In the mirror universe, emperors ruled the Terran Empire in 2155 and 2267. 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 became 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") In addition, the Klingon emperor of the House of Duras ruled the Klingon-Cardassian Alliance, but real power was in the hands of the Regent. (DS9: "Crossover", "Shattered Mirror")

On the planet 892-IV, a world identical to the Roman Empire had developed which was, as of 2268, ruled by emperors who could trace their lines back 2,000 years to their own Julius and Augustus Caesars. (TOS: "Bread and Circuses")

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")

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", and Emperor, President, First Minister, and Emissary seemed too "pretentious." (DS9: "Ties of Blood and Water")

List of emperors and empresses Edit

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