• Refers to a time series or time series process which has no natural mean. This condition is an accurate representation of many financial time series such as stocks or volatilities. This compares to Stationary.

Stationary: Refers to a time series or time series process which has a natural mean or tendency towards one. Over time and given larger samples, some economic time series tend to converge towards a natural level with stable volatilities. This compares to Nonstationary.