This term is used in two connected senses. (1) In relation to a time series it means a future value calculated according to predetermined changes in the assumptions of the environment. (2) More recently, it has been used in probability theory to denote the conditional expectation of a variate. Since a regression equation gives the expectation of the dependent variate conditional upon values of the predicted (“independent”) variates and such equations are used for forecasting or prediction, the usages are connected.