To unseat someone means to remove them from a position of power or authority, especially through a challenge or competition. When a challenger unseats a champion in boxing, they win the title belt. When voters unseat a senator, they elect someone new to take that seat in Congress.

Today, we use it more broadly. A tennis player might unseat the top-ranked player by beating them in a tournament. A new restaurant might unseat an old favorite as the best place in town.

Notice that unseating someone usually involves direct competition or confrontation. You don't unseat someone who simply retires or steps down on their own. The word suggests an active challenge: someone worked hard to take that position away. When the underdog team finally unseats the defending champions, it means they earned the victory through skill and determination, not because the champions quit.