Emperor

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 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." 

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 of Kahless the Unforgettable assumed the title of emperor, with merely symbolic power. 

In the mirror universe, emperors ruled the Terran Empire throughout the 22nd and 23rd century. In 2155, $𝜇$ declared herself empress of the Terran Empire after seizing control of the USS Defiant (NCC-1764) and ordering Starfleet's unconditional surrender. $𝜇$ had become the Terran Empire's Emperor by 2256. $𝜇$ suggested that $𝜇$ might aspire to become emperor. 

On the planet 892-IV, the was by 2268 ruled by emperors who could trace their lines back 2,000 years to their own Julius and Augustus Caesars. 

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 seemed too "pretentious." 

Earth

 * Emperor of the Roman Empire (1st century)
 * Emperor Caesar Augustus of the Roman Empire
 * Emperor Flavius Honorius of the Western Roman Empire
 * Emperor of the Qin dynasty in China (3rd century BC)
 * Napoléon Bonaparte, Emperor of the French (19th century)

Terran Empire

 * Emperor of the Terran Empire (2155)
 * Empress $𝜇$ of the Terran Empire (2155)
 * Emperor $𝜇$ of the Terran Empire (2250s)
 * Emperor of the Terran Empire (2267)

Klingon Empire

 * Emperor of the Klingon Empire (2369)
 * Emperor Kahless the Unforgettable of the Klingon Empire (9th century)
 * Emperor Mur'Eq of the Klingon Empire
 * Emperor of the Klingon Empire (Second Dynasty)
 * Emperor of the Klingon Empire
 * Emperor Sompek of the Klingon Empire

Others

 * Emperor of 892-IV (2268)
 * Empress of the Romulan Star Empire
 * Emperor of Kanda IV (Seventh Dynasty)

Imperator Empereur Keizer