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 \( [x, x +dx] \) .