1. A person who supplants or forces out someone else: Mr. Simmons was accused of being the ouster when the chairman of the committee was expelled from office.
2. The act of removing or forcing somebody out of a place or a position: The army general was involved in the ouster of the head of the country's government.
3. The expulsion or removal from a position or place being occupied by someone: The opposition party called for the ouster of the cabinet minister.