dictator

n. tyrant, despot, autocrat

A dictator is an autoritarian, often totalitarian ruler (e.g. absolutist or autocratic) who assumes sole power over his state, though the term is normally not applied to those who aquire such position by regular constitutional means, such as a hereditary absolute monarch, except to denote personal abuse of power. The word originated as the title of a magistrate in ancient Rome appointed by the Senate to rule the republic in times of emergency (see Roman dictator and justitium). Like the term tyrant, originally a respectable Ancient Greek title, and to a lesser degree autocrat, it came to be used almost exclusively as a non-titular term for oppressive, even abusive rule, yet had rare modern titular uses.
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Noun
1. a speaker who dictates to a secretary or a recording machine
(hypernym) speaker, talker, utterer, verbalizer, verbaliser
(derivation) dictate
2. a ruler who is unconstrained by law
(synonym) potentate
(hypernym) ruler, swayer
(hyponym) shogun
(derivation) dictate
3. a person behaves in an tyrannical manner; "my boss is a dictator who makes everyone work overtime"
(synonym) authoritarian
(hypernym) oppressor
(hyponym) Big Brother
(derivation) order, prescribe, dictate

n. dictator, tyrant, despot, autocrat

(n.)
One who dictates; one who prescribes rules and maxims authoritatively for the direction of others.
  

(n.)
One invested with absolute authority; especially, a magistrate created in times of exigence and distress, and invested with unlimited power.