Aristocracy is a form of government in which power is held by a select group of people. The name derives from the Greek adjective αριστος, meaning "best", and the verb κρατειν, meaning "to rule". Aristocracy was once one of the most common forms of government; present as early as 800 BC in the polis of Athens. It was used during the Middle Ages in slightly modified forms, usually in conjuncture with a monarch, throughout Europe. In his dialogue The Republic, the philosopher Plato cites the aristocratic rule of Philosopher-kings as the most just form of government. In the early Roman Republic there was an aristocratic form of democracy, with legislative powers vested in a Senate comprised of an aristocratic nobility
Usually membership of the aristocracy is hereditary, if it is gained by an act of valor or a similar act is sometimes called timocracy instead.