Bayes' theorem is a key concept in statistics and probability, named after the statistician Thomas Bayes, who first presented the idea. It explains the connection between the likelihood of an event and the factors that may influence it. In mathematical terms, it is expressed as P(A|B) = [P(B|A) * P(A)] / P(B). In this equation, P(A|B) denotes the probability of event A occurring given that event B is true. P(B|A) indicates the probability of event B occurring given that A is true, while P(A) and P(B) represent the probabilities of events A and B, respectively. A practical illustration of Bayes' theorem could be weather forecasting. For example, if it typically rains 50% of the time in your region, but the likelihood increases to 90% when dark clouds are present, observing dark clouds allows you to revise your initial 50% forecast to 90% based on this new information.