We may be interested in the probability of an event for one random variable, irrespective of the outcome of another random variable. There is no special notation for marginal probability; it is just the sum or union over all the probabilities of all events for the second variable for a given fixed event for the first variable.

$P(X=A) = \sum_{i=1}^n P(X=A,Y=y_i) = \sum_{y} P(X,Y)$