Conditional probability is the probability of an event occurring given that another event has occurred. It is denoted by P(A|B), which means the probability of event A occurring given that event B has occurred.

The formula for conditional probability is:

P(A|B) = P(A and B) / P(B)

where P(A and B) is the probability of both events A and B occurring, and P(B) is the probability of event B occurring.

For example, let's say we have a deck of cards and we want to find the probability of drawing a spade given that we have already drawn a black card. We can use conditional probability as follows:

P(Spade|Black) = P(Spade and Black) / P(Black)

The probability of drawing a spade and a black card is 13/52 (since there are 13 black spades in the deck), and the probability of drawing a black card is 26/52 (since half of the deck is black). Therefore,

P(Spade|Black) = (13/52) / (26/52) = 1/2

So the probability of drawing a spade given that we have already drawn a black card is 1/2.