In Bayesian statistics you have to represent your level of belief in terms of a probability. This not a real probability, but it does obey the Kolomogrov laws, so behaves like probability in many circumstances. Crucially, when a posterior 'probability' is calculated this too is a measure of belief informed by additional evidence.
Also used in hcistats2e: Chap. 7: pages 74, 77
Used in glossary entries: Bayesian statistics, Kolomogrov laws