Counting and Probability
n(A) = number of A P(A) = probability of A P(A’) = Probability of A not happening P(~A) = Probability of A not happening P(A|B) = Probability of B given A P(A∪B) = Probability of A and B P(A∩B) = Probability of Intersection of A and B
A could mean anything, it is a variable so it could mean: Aphids, Apples, Player A, etc. Its very open-ended.