or mathematical expectation, n. (Statistics) the sum or integral, over all possible values of a random variable, of the product of the value of the variable, or some given function of it, with the probability of that value; this is written E[ f(X)]. Thus where [a, b] is the range of values of the variable, and p(x) is its probability function. E(X) is the mean of the distribution, and is its variance. See also moment.