A tyrant was originally one who illegally seized and controlled a governmental power in a polis

Polis

Polis , plural poleis , literally means city in Greek. It could also mean citizenship and body of citizens. In modern historiography "polis" is normally used to indicate the ancient Greek city-states, like Classical Athens and its contemporaries, so polis is often translated as "city-state."The...

. Tyrants were a group of individuals who took over many Greek poleis during the uprising of the middle classes in the sixth and seventh centuries BC, ousting the aristocratic governments.

Plato

Plato

Plato , was a Classical Greek philosopher, mathematician, student of Socrates, writer of philosophical dialogues, and founder of the Academy in Athens, the first institution of higher learning in the Western world. Along with his mentor, Socrates, and his student, Aristotle, Plato helped to lay the...

 and Aristotle

Aristotle

Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

 define a tyrant as, "one who rules without law, looks to his own advantage rather than that of his subjects, and uses extreme and cruel tactics—against his own people as well as others".

In common usage, the word "tyrant" carries connotations of a harsh and cruel ruler who places his or her own interests or the interests of an oligarchy

Oligarchy

Oligarchy is a form of power structure in which power effectively rests with an elite class distinguished by royalty, wealth, family ties, commercial, and/or military legitimacy...

 over the best interests of the general population, which the tyrant governs or controls.