Conditional probability is a measure of the likelihood of an event occurring given that another event has already occurred. It is denoted as \( P(A | B) \), which reads "the probability of event A given event B." Mathematically, conditional probability can be defined using the formula: \[ P(A | B) = \frac{P(A \cap B)}{P(B)} \] provided that \( P(B) > 0 \).