In common usage, saving generally means putting money aside, for example, by putting money in the bank or investing in a pension plan. Outside of economics, saving is typically used to refer to economizing, cutting costs, or to rescuing someone or something.

In economics, personal saving has been defined as personal disposable income minus personal consumption expenditure. In other words, income that is not consumed by immediately buying goods and services is saved. Other kinds of saving can occur, as with corporate retained earnings (profits minus dividend and tax payments) and a government budget surplus.

There is some disagreement about what counts as saving. For example, the part of a person's income that is spent on mortgage repayments is not spent on present consumption and is therefore saving by the above definition, even though people do not always think of repaying a loan as saving. However, in the U.S. measurement of the numbers behind its gross national product (i.e., the National Income and Product Accounts), personal interest payments are not treated as "saving" unless the institutions and people who receive them save them.

"Saving" differs from "savings." The former refers to an increase in one's assets, an increase in net worth, whereas the latter refers to one part of one's assets, usually deposits in savings accounts, or to all of one's assets. Saving refers to an activity occurring over time, a flow variable, whereas savings refers to something that exists at any one time, a stock variable.

Investment

Saving is closely related to investment. By not using income to buy consumer goods and services, it is possible for resources to instead be invested by being used to produce fixed capital, such as factories and machinery. Saving can therefore be vital to increase the amount of fixed capital available, which contributes to economic growth.

However, increased saving does not always correspond to increased investment, since the saving and investment decisions are made by different groups (households, businesses) and for different reasons. This means that saving may increase without increasing investment, possibly causing a short-fall of demand (a pile-up of inventories, a cut-back of production, employment, and income, and thus a recession) rather than to economic growth. (This is often called the "paradox of thrift.") If saving falls below investment, on the other hand, it can lead to a growth of aggregate demand and an economic boom. These statements are conditional since aggregate demand consists of more than investment and consumption (non-saving).

Interest rates

Classical economics posited that interest rates would adjust to equate saving and investment, avoiding a pile-up of inventories (general overproduction). A rise in saving would cause a fall in interest rates, stimulating investment. But Keynes argued that neither saving nor investment were very responsive to interest rates (i.e., that both were interest inelastic) so that large interest rate changes were needed. Further, it was the demand for and supplies of stocks of money that determined interest rates in the short run. Thus, saving could exceed investment for significant amounts of time, causing a general glut and a recession.