Epistemic contextualism (EC) is a recent and hotly debated position. In its dominant form, EC is the view that the proposition expressed by a given knowledge sentence (‘S knows that p’, ‘S doesn't know that p’) depends upon the context in which it is uttered. What makes this view interesting and controversial is that ‘context’ here refers, not to certain features of the putative subject of knowledge or his/her objective situation, but rather to features of the knowledge attributor(s)' psychology and/or conversational-practical situation.
For instance - Suppose we are interested in whether Jones, an ordinary non-medically trained person, has the general information that polio is caused by a virus. If his response to our question is that he remembers the paper reporting that Salk said it was, then this is good enough. He has performed adequately given the issue-context. But suppose the context is an examination for the M. D. degree. Here we expect a lot more. If the candidate simply said what Jones did, we would take him as being very deficient in knowledge. Thus relative to one issue-context a person may be justified in believing, but not justified relative to another context.
See the full article at http://plato.stanford.edu/entries/contextualism-epistemology/