The co-movement of two or more non-stationary variables over time. If two variables are cointegrated, regression of one variable on the other results in a set of residuals that is stationary. Existence of this long-run equilibrium relationship allows one to impose parameter restrictions on a Vector AutoRegressive model (VAR). The restricted VAR can be expressed in various ways, one of which is the error correction model. With more than two non-stationary variables, it is possible to have more than one long-run equilibrium relationship among them.