autocrat

n. ruler with unlimited authority, dictator

An autocracy is a form of government in which the political power is held by a single self appointed ruler, usually a dictator. The term autocrat is derived from the Greek word autokratôr (lit. "self-ruler", or to: "rule by one's self"). Compare with oligarchy (literally means rule by the few) and democracy (rule by the people).
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Noun
1. a cruel and oppressive dictator
(synonym) tyrant, despot
(hypernym) dictator, potentate
(hyponym) czar

(a.)
One who rules with undisputed sway in any company or relation; a despot.
  

(a.)
An absolute sovereign; a monarch who holds and exercises the powers of government by claim of absolute right, not subject to restriction; as, Autocrat of all the Russias (a title of the Czar).
  

autocrat