a theorem establishing a method of calculating conditional (adjective) statistical probabilities, in which a known probability is modified in the light of later events that affect the data under consideration (Ex.: the probability of selecting a heart from a deck of cards is 13/52; if a card is selected from a full deck and that card is a heart, the probability that the next card selected will also be a heart becomes 12/51, but if that first selected card is not a heart, the probability for the next card becomes 13/51)

Origin of Bayes' theorem

after T.*Bayes*(1702-61), Eng theologian and mathematician known for his early work in modern probability theory