This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. a belief which, if proved to be true, would imply outright or indirectly that another belief were false

"Once I see the connection, then I have a defeater: the defeater is the conjunction of those propositions, together with the proposition that if that conjunction is true, then you weren't at the game at 9: 30."

defeated