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.
Tyrant vs Caliph
In the Islamic concept of tyrant, a tyrant is any person or man-made system of law such as monarchy, constitutional government, or republic and its elected representatives, who disavow Allah and his Caliphate, empowered to enforce Allah's will as spoken through the mouth of the Prophet Muhammad and reiterated in the Koran and Hadiths. All other systems of law are man-made tyrannies to be opposed, fought against, and overthrown, per Allah.
The Earth and Its Peoples A Global History, Bulliet et al., 2005.