Anon: well-calibrated means roughly that in the class of all events you think have probability p to being true, the proportion of them that turn out to be true is p.

More formally, suppose you have a probability distribution over something you are going to observe. If the log probability of the event which actually occurs is equal to the entropy of your distribution, you are well calibrated. If it is above you are over confident, if it is below you are under confident. By this measure, assigning every possibility equal probability will always be calibrated.

This is related to relative entropy.