This is the probability of an event given (or conditional upon) the occurrence of another event, e.g. the probability of disease Y given exposure to factor X.
The probability, written Pr(A|B), of event A given that event B has occurred.
Conditional probability is the probability of an event occurring given that another event also occurs. It is expressed as P(A/B). It reads "Probability of Event A on condition of Event B." P(A/B) = P(A and B)/P(B), where P(B) is the probability of event B and P(A and B) is the joint probability of A and B.