Macroeconomic Usages
The usage of 'long run' and 'short run' in macroeconomics differs somewhat from the above microeconomic usage. J.M. Keynes (1936) emphasized fundamental factors of a market economy that might result in prolonged periods away from full-employment. In later macro usage, the long run is the period in which the price level for the economy is completely flexible as to shifts in aggregate demand and aggregate supply. In addition there is full mobility of labor and capital between sectors of the economy and full capital mobility between nations. In the short run none of these conditions need fully hold. The price is sticky or fixed as to changes in aggregate demand or supply, capital is not fully mobile between sectors, and capital is not fully mobile to interest rate differences among countries & fixed exchange rates.
A famous critique of neglecting short-run analysis was by John Maynard Keynes, who wrote that "In the long run, we are all dead," referring to the long-run proposition of the quantity theory of, for example, a doubling of the money supply doubling the price level.
Read more about this topic: Long Run And Short Run