1. a person or thing that defeats someone or something
2. something that prevents success or causes failure

Examples

• The young boxer proved to be the defeater of the longtime champion.
• Rain was the defeater of our picnic plans.
• She emerged as the defeater in the chess tournament.
• Poor planning was the defeater of the project.
• The underdog team became the defeater of the favorites.
• Lack of funding was the main defeater of the proposal.