Meaning of confidence factor in J48

Question

I try to use J48 classifier from RWeka library in R (C4.5 algorithm). I can parametrize this classifier with C parameter which means 'confidence factor'. What does this value exactly mean? I know that bigger value means that I believe more my learning set is a good representation of the whole population and the algorithm will be less likely to prune. But what it exactly means? Is there any formula how to interpret this value?