This is the probability of one event (say A) given knowledge of another event (B). It is usually written as P(A|B), read as "the probability of A given B". For example, while the probability that it will rain on a typical day might be 50%, on a cloudy day this might be 80%; that is P(rain|cloudy) = 0.8.
Also used in hcistats2e: Chap. 5: pages 61, 62, 63, 64; Chap. 7: pages 76, 79