To oust means to force someone out of a position, place, or job, often suddenly. When a company ousts its CEO, the board of directors removes that person from leadership. When citizens oust a corrupt mayor through an election, they vote that person out of office.

The word carries a sense of pushing someone out who doesn't want to leave. You wouldn't say someone was ousted from a job they quit voluntarily. Instead, oust suggests removal by force, vote, or pressure. A dictator might be ousted by a revolution. A sports team might oust the defending champions by beating them in the playoffs, knocking them out of the tournament.

Notice that oust often implies the person being removed has been holding their position for a while. Someone needs to be in before they can be ousted. You can oust a leader, oust an incumbent politician, or oust someone from power, but you wouldn't use the word for someone who just arrived. When a rebellion succeeds in ousting a tyrant, it means they've finally removed someone who had been ruling for some time.