Dionysius I or Dionysius the Elder was a Greek tyrant of Syracuse, Sicily. He conquered several cities in Sicily and southern Italy, opposed Carthage's influence in Sicily and made Syracuse the most powerful of the Western Greek colonies. He was regarded by the ancients as an example of the worst kind of despot: cruel, suspicious, and vindictive.

A tyrant, in the modern English usage of the word, is an absolute ruler who is unrestrained by law, or one who has usurped a legitimate ruler's sovereignty. Often portrayed as cruel, tyrants may defend their positions by resorting to repressive means. The original Greek term meant an absolute sovereign who came to power without constitutional right, yet the word had a neutral connotation during the Archaic and early Classical periods. However, Greek philosopher Plato saw tyrannos as a negative word, and on account of the decisive influence of philosophy on politics, deemed tyranny the "fourth and worst disorder of a state."Tyrants lack "the very faculty that is the instrument of judgment"—reason. The tyrannical man is enslaved because the best part of him (reason) is enslaved, and likewise, the tyrannical state is enslaved, because it too lacks reason and order.