In epistemology, a defeater is a belief B1 that is held to be incompatible with another belief B2, hence arguments or evidence supporting B1 can be used to refute B2.
An undercutting defeater is B1 such that B1 does not oppose B2, but rather that the ramifications of B1, were it possible to obtain, cast doubt on the premises for B2.
An opposing defeater is B1 such that B1 has a factual or otherwise claim that, were it to be obtained, would falsify B2.
References
Defeater Wikipedia(Text) CC BY-SA