(covers information from several alternate timelines)
A tyrant was the absolute ruler of a political entity or specific area who ruled without constraints and oppressive towards his subjects.
During the Eugenics Wars on Earth in the 1990s, a series of Augment tyrants ruled over several areas. Khan Noonien Singh was described as "the best of the tyrants" and was the last one to be overthrown. (TOS: "Space Seed")