Rule by a small number of people; in Aristotle's classification, the deviant form of rule by the few, in which the wealthy or well-born rule in their own interest.
In more detail
Aristotle classed constitutions by the number of rulers and by whether they aim at the common advantage: oligarchy is the deviant counterpart of aristocracy. In modern political science the term also covers the concentration of power within organisations, as in Michels's iron law of oligarchy.