noun (plural aristocracies)[treated as singular] (or plural, usually the aristocracy)
late 15th century: from Old French aristocratie, from Greek aristokratia, from aristos 'best' + -kratia 'power'. The term originally denoted the government of a state by its best citizens, later by the rich and well-born, hence the sense 'nobility', regardless of the form of government (mid 17th century)
Aristocracy, oligarchy, and plutocracy are sometimes confused. All mean some form of rule by a small elite. Aristocracy is rule by a traditional elite, held to be made up of ‘the best’ people, and is usually hereditary. Oligarchy is literally rule by a few. Plutocracy is rule by the (necessarily few) very rich.