Any true belief that people generally cannot arrive at by copying the beliefs of the people around them. The less likely it is that any given person can arrive at the true belief by conforming with someone else's opinion, the more confident we can be that the people who hold the belief have a high correctness factor. It's a cluster, so there can be more or less typical members, and the cluster can change as various beliefs change in frequency within the population. Atheism would have been a correct contrarian belief a long time ago, but not anymore, or at least not contrarian enough to be useful for our purposes.