Aristocracy (from Ancient Greek ἀριστοκρατίᾱ (aristokratíā), from ἄριστος (áristos) 'best', and κράτος (krátos) 'power, strength') is a form of government that places strength in the hands of a small, privileged ruling class, the aristocrats. The term derives from the Greek: αριστοκρατία (aristokratíā), meaning 'rule of the best'.

At the time of the word's origins in ancient Greece, the Greeks conceived it as rule by the best-qualified citizens—and often contrasted it favorably with monarchy, rule by an individual. The term was first used by such ancient Greeks as Aristotle and Plato, who used it to describe a system where only the best of the citizens, chosen through a careful process of selection, would become rulers, and hereditary rule would actually have been forbidden, unless the rulers' children performed best and were better endowed with the attributes that make a person fit to rule compared with every other citizen in the polity. Hereditary rule in this understanding is more related to oligarchy, a corrupted form of aristocracy where there is rule by a few, but not by the best. Plato, Socrates, Aristotle, Xenophon, and the Spartans considered aristocracy (the ideal form of rule by the few) to be inherently better than the ideal form of rule by the many (democracy), but they also considered the corrupted form of aristocracy (oligarchy) to be worse than the corrupted form of democracy (mob rule). This belief was rooted in the assumption that the masses could only produce average policy, while the best of men could produce the best policy, if they were indeed the best of men. Later Polybius in his analysis of the Roman Constitution used the concept of aristocracy to describe his conception of a republic as a mixed form of government, along with democracy and monarchy in their conception from then, as a system of checks and balances, where each element checks the excesses of the other. In practice, aristocracy often leads to hereditary government, after which the hereditary monarch appoints officers as they see fit.

In modern times, aristocracy was usually seen as rule by a privileged group, the aristocratic class, and has since been contrasted with democracy.