Full Idea
The 'defeasibility' approach says that having knowledge requires, in addition to justified true belief, there being no true information which, if learned, would result in the person in question no longer being justified in believing the claim.
Gist of Idea
The 'defeasibility' approach says true justified belief is knowledge if no undermining facts could be known
Source
Jonathan Kvanvig (Truth is not the Primary Epistemic Goal [2005], 'Epistemic')
Book Reference
'Contemporary Debates in Epistemology (2nd ed)', ed/tr. Steup/Turri/Sosa [Wiley Blackwell 2014], p.355
A Reaction
I take this to be an externalist view, since it depends on information of which the cognizer may be unaware. A defeater may yet have an undiscovered counter-defeater. The only real defeater is the falsehood of the proposition.