Variable

An element in a model. For example, in the model RS&P_t+1 = a + b Tbill _t + e_t, where RS&P_t+1 is the return on the S&P in month _t+1 and Tbill is the Tbill return at month _t, both RS&P and Tbill are "variables" because they change through time; i.e., they are not constant.