In probability, conditional probability is the probability that an event occurs given another (conditioning) event occurred.

In probability, conditional probability is the probability that an event occurs given something else has already occurred. The probability of an event $A$ given another event $B$ is written as $\mathbb P(A|B)$, and is related to the marginal and joint probabilities via $$\mathbb P(A|B)=\frac{\mathbb P(A\cap B)}{\mathbb P(B)}.$$ The Bayesian approach often relies on Bayes' rule, which relates $\mathbb P(A|B)$ and $\mathbb P(B|A)$ via $$\mathbb P(A|B)=\frac{\mathbb P(B|A)\mathbb P(A)}{\mathbb P(B)}.$$ More on Wikipedia