An important distinction exists between prospective and retrospective methods of calculating economic quantities such as incomes, savings, and investments; and... a corresponding distinction of great theoretical importance must be drawn between two alternative methods of defining these quantities. Quantities defined in terms of measurements made at the end of the period in question are referred to as ex post; quantities defined in terms of action planned at the beginning of the period in question are referred to as ex ante.

p. 34 - Monetary Equilibrium, (1939)