In Bayesian statistics the 'probabilities' used for the prior distribution and hence the resulting posterior distribution are encoding (belief) as probability – that is using a number between 0 and 1 to represent a level of belief. To distinguish these from true probabilities they are called plausibility or reasonable expectation. For the purposes of mathematical calculation, plausibility satisfies the Kolomogrov laws of probability and so can be treated mathematically in the same way even though its meaning is subtly different.
Used on page 8
Also known as reasonable expectation