A moderate government may, whenever it pleases, and without the least danger, relax its springs: it supports itself by the laws and by its own internal strength. But, when a despotic prince ceases one single moment to lift up his arm, when he cannot instantly demolish those whom he has intrusted with the first employments, all is over: for, as fear, the spring of this government, no longer subsists, the people are left without a protector.
—  Montesquieu, The Spirit of Laws, Part I, Book 3, Chapter 9

In vulgar usage, a tyrant is a king who governs violently and without regard for justice and law. In the exact sense, a tyrant is an individual who arrogates to himself the royal authority without having a right to it. This is how the Greeks understood the word “tyrant”: they applied it indifferently to good and bad princes whose authority was not legitimate. Tyrant and usurper are thus perfectly synonymous terms. In order that I may give different things different names, I call him who usurps the royal authority a tyrant, and him who usurps the sovereign power a despot. The tyrant is he who thrusts himself in contrary to the laws to govern in accordance with the laws; the despot is he who sets himself above the laws themselves. Thus the tyrant cannot be a despot, but the despot is always a tyrant.
—  Jean-Jacques Rousseau, The Social Contract, Book III, Chapter 10