A political system in which power is held by a small group of people. Often used in political science or historical contexts.
That country is ruled by an oligarchy.
民主制 is democracy, where power is held by the people, contrasting with the elite rule of 寡頭制.
From 寡頭 (few heads) + 制 (system). The term is a direct translation of the Western political concept 'oligarchy'.