Investment is the amount of goods purchased or accumulated per unit time which are not consumed at the present time. The types of investment are residential investment in housing that will provide a flow of housing services over an extended time, non-residential fixed investment in things such as new machinery or factories, human capital investment in workforce education, and inventory investment (the accumulation, intentional or unintentional, of goods inventories).
In measures of national income and output, "gross investment" (represented by the variable I ) is a component of gross domestic product (GDP), given in the formula GDP = C + I + G + NX, where C is consumption, G is government spending, and NX is net exports, given by the difference between the exports and imports, X − M. Thus investment is everything that remains of total expenditure after consumption, government spending, and net exports are subtracted (i.e. I = GDP − C − G − NX ).
Fixed investment, as expenditure over a period of time (e.g., "per year"), is not capital but rather leads to changes in the amount of capital. The time dimension of investment makes it a flow. By contrast, capital is a stock—that is, accumulated net investment up to a point in time.