A challenger is someone who competes against another person, especially when that other person already holds a position, title, or championship. When a new boxer steps into the ring to face the reigning champion, that new boxer is the challenger. When a candidate runs against the current mayor in an election, they're the challenger trying to unseat the incumbent.

The word comes from the idea of issuing a challenge, which is a call to compete or prove yourself. A challenger specifically tries to defeat someone who already holds what they want. In chess tournaments, the challenger faces the current champion. In business, a challenger brand tries to compete with the established market leader.

Being a challenger often means you're the underdog, but it also means you're brave enough to test yourself against the best. History remembers many great challengers who eventually became champions themselves. The word can also describe someone who questions or opposes ideas and assumptions: a student who respectfully challenges their teacher's thinking in a class discussion, or a scientist who challenges old theories with new evidence.