A probability distribution that assigns equal probability to every value of a random variable. In other words, given a discrete random variable A, P(A=x) = P(A=y) for all possible values x and y of A; for a continuous random variable A, the probability density function P(x) = 1/(b-a) for a < x < b, 0 otherwise.

Almost all random number generators (or pseudo-random number generators, if you prefer) attempt to generate a uniform distribution because other distributions can usually be defined in terms of them. For example, a Gaussian distribution can be generated from a uniform distribution using the Box-Muller transformation.