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

Across Europe, the aristocracy exercised immense economic, political, and social influence. In Western Christian countries, the aristocracy was mostly equal with magnates, also known as the titled or higher nobility, however the members of the more numerous social class, the untitled lower nobility (petty nobility or gentry) were not part of the aristocracy.

In ancient Greece, the Greeks conceived aristocracy 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 (politeia), 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 modern times, aristocracy was usually seen as rule by a privileged group, the aristocratic class, and has since been contrasted with democracy.

The concept evolved in ancient Greece in which a council of leading citizens was commonly empowered. That was contrasted with representative democracy in which a council of citizens was appointed as the "senate" of a city state or other political unit. The Greeks did not like the concept of monarchy, and as their democratic system fell, aristocracy was upheld.

According to Tomás Fernández de Medrano, a just and virtuous form of republic is known as aristocracy, meaning the rule of the most virtuous, referred to in Latin as Optimates because they are regarded as good and honorable. In his 1602 political treatise República Mista, Medrano explains how this system arises when a select group of men, distinguished by their virtue, morality, and wisdom, hold authority over the rest, whether broadly or in specific matters, governing solely for the benefit and welfare of the public. A prime example of this was seen in Sparta, whose exceptional governance allowed it to rule Greece for nearly five centuries.

Plato's concept of aristocracy envisions an ideal state governed by a philosopher-king—a ruler who possesses wisdom and a love for truth. He defines these "philosopher-kings" as individuals who "love the sight of truth." To illustrate this idea, Plato uses analogies such as a captain steering a ship and a doctor administering medicine, emphasizing that just as not everyone is naturally suited to navigation or medicine, not everyone is fit to govern. A significant portion of the Republic is then dedicated to outlining the educational system necessary to cultivate philosopher-kings.