Tyranny is a noun that describes a repressive and arbitrarily cruel regime. Don't accuse your mother of tyranny just because she won't let you play video games all weekend long.
One of the root words of tyranny is the Latin tyrannia which means the "rule of a tyrant" and a tyrant is a "cruel master." Today, it can mean the repressive regime of a dictator or it can also mean being controlled by something metaphorically harsh like time or work. Maybe the oldest example of a tyranny is that of Tyrannosaurus Rex, or "king of the giant lizards," who many of us grew up thinking was the largest, most predatory and frightening of the dinosaurs who supposedly ruled over the rest of the dinosaur world with its huge, sharp teeth and claws.