A tyrant (Greek: τύραννος - tyrannos) is a term the Greeks used to describe a person who seized and held power in violation of the normal laws and traditions of the community. Tyrants appeared in many Greek city states in the seventh and sixth centuries B.C. often taking advantage of the disaffection of the middle class against the old elite.
Today, the word 'tyrant' is used to refer to an unjust and autocratic ruler, who enriches himself or satisfies his lust for power at the expense of his people. Examples include Nicolae Ceausescu in Romania and Robert Mugabe of Zimbabwe in his later years.
The Earth and Its Peoples A Global History, Bulliet et al, 2005.