A tyrant (from Ancient Greek τύραννος (túrannos) 'absolute ruler'), in the modern English usage of the word, is an absolute ruler who is unrestrained by law, or one who has usurped a legitimate ruler's sovereignty. Often portrayed as cruel, tyrants may defend their positions by resorting to repressive means. The original Greek term meant an absolute sovereign who came to power without constitutional right, yet the word had a neutral connotation during the Archaic and early Classical periods. However, Greek philosopher Plato saw tyrannos as a negative form of government, and on account of the decisive influence of philosophy on politics, deemed tyranny the "fourth and worst disorder of a state."

Tyrants lack "the very faculty that is the instrument of judgment"—reason. The tyrannical man is enslaved because the best part of him (reason) is enslaved, and likewise, the tyrannical state is enslaved, because it too lacks reason and order.

The philosophers Plato and Aristotle defined a tyrant as a person who rules without law, using extreme and cruel methods against both his own people and others. The Encyclopédie defined the term as a usurper of sovereign power who makes "his subjects the victims of his passions and unjust desires, which he substitutes for laws". In the late fifth and fourth centuries BC, a new kind of tyrant, one who had the support of the military, arose – specifically in Sicily.

One can apply accusations of tyranny to a variety of types of government:

The English noun tyrant appears in Middle English use, via Old French, from the 1290s. The word derives from Latin tyrannus, meaning 'illegitimate ruler', and this in turn from the Greek τύραννος túrannos 'monarch, ruler of a polis'; túrannos is of Pre-Greek origin, perhaps from Lydian. The final -t arises in Old French by association with the present participles in -ant.

"The word 'tyranny' is used with many meanings, not only by the Greeks but throughout the tradition of the great books." The Oxford English Dictionary offers alternative definitions: a ruler, an illegitimate ruler, an absolute ruler, or an oppressive, unjust, or cruel ruler. The term is usually applied to vicious autocrats who rule their subjects by brutal methods. Scholar Paul Rahe has asserted that tyrannical rule "in one form or another" has "been the norm throughout human history", and added that "it is not apt to disappear."

Ancient Greek and Sicilian tyrants were influential opportunists that came to power by securing the support of different factions of a deme. The word tyrannos, possibly pre-Greek, Pelasgian or eastern in origin, then carried no ethical censure; it simply referred to anyone, good or bad, who obtained executive power in a polis by unconventional means. Support for the tyrants could come from fellow oligarchs, from the growing middle class or from the peasants who had no land or were in debt to the wealthy landowners.

The Greek tyrants stayed in power by using mercenary soldiers from outside of their respective city-state. To mock tyranny, Thales wrote that the strangest thing to see is "an aged tyrant", meaning that tyrants do not have the public support to survive for long.