challenger
Definitions
A person or group that competes against or challenges someone else, especially in a contest or election.
A person who questions the validity, truth, or legality of something.
Examples
The young challenger surprised everyone by defeating the reigning champion in the final round.
As the primary challenger to the incumbent mayor, she focused her campaign on improving public transportation.
The new software is a serious challenger to the industry leader, offering more features at a lower price.
Synonyms
contender competitor rival opponent adversary aspirant candidate