Accessibilism is a form of internalism in epistemology. It is the position that whether someone's belief is justified supervenes only on facts to which that person has some sort of access. There are a variety of different views as to what form this access must take. The most common position is that it need only involve reflection.
Accessibilism is sometimes, but not always, associated with the position that to have a justified belief one must be in a position to have a justified belief that one has that justified belief.