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 {x} for X may be discrete, or continuous. If the set of values is continuous, then wX(x) is a probability density so that wX(x)dx is the probability that one finds the stochastic variable X to have values in the range [x,x+dx] .