Full Idea
Internalist theories make justifiability of a belief a function of the internal states of the believer, in the sense that if we vary anything but his internal states the justifiability of the belief does not vary.
Gist of Idea
Internalism says if anything external varies, the justifiability of the belief does not vary
Source
J Pollock / J Cruz (Contemporary theories of Knowledge (2nd) [1999], §5.4.3)
Book Reference
Pollock,J.L./Cruz,J: 'Contemporary Theories of Knowledge (2nd)' [Rowman and Littlefield 1999], p.137
A Reaction
This seems to be a nice clear definition of internalism (and, by implication, externalism). It favours externalism. I know my car is in the car park; someone takes it for a joyride, then replaces it; my good justification seems thereby weakened.