Conditional probability is the likelihood of an event occurring given that another event has already occurred. It is expressed mathematically as P(A | B), which reads as "the probability of event A occurring given that event B has occurred." This concept helps in understanding how the occurrence of one event can influence the probability of another. For example, consider a deck of cards. The probability of drawing an Ace is 4 out of 52. However, if we know that the card drawn is a spade, the conditional probability of drawing an Ace becomes 1 out of 13, since there is only one Ace of spades in the 13 spades.