Reliable

is a charactheristic of a forecast system for which the probabilities issued for a specific event very a proportion of times equal to the climatological frequency of the event. A reliable system which predicts, for example  50% (or 20%,  or 73%) probability of rain ,should, on averge,  be correct 50% (or 20%,  or 73%)  of the times, no more, no less.