a system of administration in which a small group of people control an organization, institution, or country, often for selfish or corrupt purposes, or the small group of people that holds control in such a system;
e.g. The country is ruled by an oligarchy, which does very little for the people's benefit. He is a member of the oligarchy of the country.
(noun)
an organization, institution, or country that is ruled/controlled by a small group of people, typically for selfish or corrupt reasons;
e.g. The country has changed from a democracy to an oligarchy over time.