Stationary process

A stationary process is one in which the probability density function of some random variable X does not change over time or position. As a result, the statistics, including the mean and variance, also do not change over time or position.

As an example, the measurement of white noise is stationary: the statistics of your measurement are unchanging. Alternatively, the measurement of a cymbal clashing is not stationary. Although a cymbal clash is basically white noise, the measurement of that noise varies over time: Before the clash, there is silence, and after the clash, the noise gradually diminishes.

Stationarity is used as a tool in time series analysis.