A stochastic process is simply a function of two variables, one is the time, the other is a stochastic variable X , defined by specifying
The set of values \left\{x\right\} for X may be discrete, or continuous. If the set of values is continuous, then w_X (x) is a probability density so that w_X (x)dx is the probability that one finds the stochastic variable X to have values in the range [x, x +dx] .