noun. A person or thing that defeats or conquers someone or something. 'This is a potential defeater for my being rational in accepting theistic belief. '

What is a defeater in philosophy?

Epistemology is concerned with justification—whether a given belief is justified or not—and related issues. ... Evidence that counts against a belief is called a defeater of this belief.

What is Defeits?

to overcome in a contest, election, battle, etc.; prevail over; vanquish: They defeated the enemy. She defeated her brother at tennis. to frustrate; thwart. to eliminate or deprive of something expected: The early returns defeated his hopes of election.