tyrant

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An absolute ruler who governs without restrictions.
2. n. A ruler who exercises power in a harsh, cruel manner.
3. n. An oppressive, harsh, arbitrary person.

Century Dictionary and Cyclopedia

1. n. In ancient Greece, an irresponsible chief or magistrate with unlimited powers, owing his office primarily to insurrection or usurpation. The first tyrants, so called, were generally the leaders of risings against the oligarchies during the seventh and sixth centuries b. c. They ruled with the popular consent in nearly all the Greek states and colonies at one time or another, transmitting their power to their heirs until democracies or new oligarchies overthrew them. Others raised themselves to the position by direct conquest or conspiracy. The arbitrary government of the tyrants was sometimes beneficent, but more often extremely oppressive and cruel. The typical tyrant in the latter sense of the word was Dionysius the Elder, of Syracuse (405–367 b. c.).
2. n. Hence A wilfully arbitrary monarch or person in authority; a ruler or master who uses his power cruelly or oppressively; any person who treats those bound to him in any way as slaves to his will; an autocratic oppressor.
3. n. A tyrannical or compulsory influence; something that constrains the will inexorably; an overruling power.
4. n. In ornithology, a tyrant-flycatcher; one of the Tyrannidæ.
5. To tyrannize over.
6. To play the tyrant; tyrannize: sometimes with indefinite it.

Wiktionary

1. n. An absolute ruler who governs without restriction.
2. n. A harsh and cruel ruler.
3. n. An oppressive, cruel and harsh person.
4. v. obsolete To act like a tyrant; to be tyrannical.

GNU Webster's 1913

1. n. An absolute ruler; a sovereign unrestrained by law or constitution; a usurper of sovereignty.
2. n. Specifically, a monarch, or other ruler or master, who uses power to oppress his subjects; a person who exercises unlawful authority, or lawful authority in an unlawful manner; one who by taxation, injustice, or cruel punishment, or the demand of unreasonable services, imposes burdens and hardships on those under his control, which law and humanity do not authorize, or which the purposes of government do not require; a cruel master; an oppressor.
3. n. (Zoöl.) Any one of numerous species of American clamatorial birds belonging to the family Tyrannidæ; -- called also tyrant bird.
4. v. obsolete To act like a tyrant; to play the tyrant; be to tyrannical.

WordNet 3.0

1. n. any person who exercises power in a cruel way
2. n. in ancient Greece, a ruler who had seized power without legal right to it
3. n. a cruel and oppressive dictator