The concept of epistemic defeat or defeasibility has come to occupy an important place in contemporary epistemology, especially in relation to the closely allied concepts of justified belief, warrant, and knowledge. These allied concepts signify positive epistemic appraisal or positive epistemic status. As a first approximation, defeasibility refers to a belief’s liability to lose some positive epistemic status, or to having this status downgraded in some particular way.
For example, a person may be epistemically justified in believing some proposition p at one time, but then the belief might become less justified or even unjustified at some later time. Moreover, beliefs may also be prevented from having or acquiring some positive epistemic status in the first place. So more generally, defeasibility refers to a kind of epistemic liability or vulnerability, the potential of loss, reduction, or prevention of some positive epistemic status. A defeater is, broadly speaking, a condition that actualizes this potential.
It’s also possible to have defeaters for your defeaters. As Alvin Plantinga points out, given the instigation of the Holy Spirit in an individual’s life, moving them to accept the great things of the gospel, the strength of these resulting beliefs is strong enough to defeat all other potential defeaters of belief in God or the great things of the gospel (e.g., justification by faith, atonement, etc.).