tyrant

n. despot, dictator, autocrat

In modern usage a tyrant is a single ruler holding vast, if not absolute power through a state or in an organization. The term carries connotations of a harsh and cruel ruler who places his/her own interests or the interests of a small oligarchy over the best interests of the general population which they govern or control. This mode of rule is referred to as tyranny. Many individual rulers or government officials are accused of tyranny, with the label almost always a matter of controversy.
See more at Wikipedia.org...

Noun
1. a cruel and oppressive dictator
(synonym) autocrat, despot
(hypernym) dictator, potentate
(hyponym) czar
(derivation) tyrannize, tyrannise, grind down
2. in ancient Greece, a ruler who had seized power without legal right to it
(hypernym) ruler, swayer
(hyponym) Dionysius, Dionysius the Elder
3. any person who exercises power in a cruel way; "his father was a tyrant"
(hypernym) person, individual, someone, somebody, mortal, human, soul

(v. i.)
To act like a tyrant; to play the tyrant; to tyrannical.
  

(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.
  

(n.)
Any one of numerous species of American clamatorial birds belonging to the family Tyrannidae; -- called also tyrant bird.
  

(n.)
An absolute ruler; a sovereign unrestrained by law or constitution; a usurper of sovereignty.
  

TYRANNY, TYRANT - The violation of those laws which regulate the division and the exercises of the sovereign power of the state. It is a violation of its Constitution.

TYRANT - The chief magistrate of the state, whether legitimate or otherwise, who violates the Constitution to act arbitrarily contrary to justice.

The term tyrant and usurper, are sometimes used as synonymous, because usurpers are almost always tyrants; usurpation is itself a tyrannical act, but properly speaking, the words usurper and tyrant convey different ideas. A king may become a tyrant, although legitimate, when he acts despotically; while a usurper may cease to be a tyrant by governing according to the dictates of justice.

This term is sometimes applied to persons in authority who violate the laws and act arbitrarily towards others.
   

This entry contains material from Bouvier's Legal Dictionary, a work published in the 1850's.