The gross domestic product (GDP) or gross domestic income (GDI) is a measure of a country's overall economic output. It is the market value of all final goods and services made within the borders of a country in a year. It is often positively correlated with the standard of living, though its use as a stand-in for measuring the standard of living has come under increasing criticism and many countries are actively exploring alternative measures to GDP for that purpose.

GDP can be determined in three ways, all of which should in principle give the same result. They are the product (or output) approach, the income approach, and the expenditure approach.

The most direct of the three is the product approach, which sums the outputs of every class of enterprise to arrive at the total. The expenditure approach works on the principle that all of the product must be bought by somebody, therefore the value of the total product must be equal to people's total expenditures in buying things. The income approach works on the principle that the incomes of the productive factors ("producers," colloquially) must be equal to the value of their product, and determines GDP by finding the sum of all producers' incomes.

Example: the expenditure method:

GDP = private consumption + gross investment + government spending + (exports − imports), or

In the name "Gross Domestic Product," "Gross" means that GDP measures production regardless of the various uses to which that production can be put. Production can be used for immediate consumption, for investment in new fixed assets or inventories, or for replacing depreciated fixed assets. If depreciation of fixed assets is subtracted from GDP, the result is called the Net domestic product; it is a measure of how much product is available for consumption or adding to the nation's wealth. In the above formula for GDP by the expenditure method, if net investment (which is gross investment minus depreciation) is substituted for gross investment, then net domestic product is obtained.

"Domestic" means that GDP measures production that takes place within the country's borders. In the expenditure-method equation given above, the exports-minus-imports term is necessary in order to null out expenditures on things not produced in the country (imports) and add in things produced but not sold in the country (exports).

Economists (since Keynes) have preferred to split the general consumption term into two parts; private consumption, and public sector (or government) spending. Two advantages of dividing total consumption this way in theoretical macroeconomics are:

• Private consumption is a central concern of welfare economics. The private investment and trade portions of the economy are ultimately directed (in mainstream economic models) to increases in long-term private consumption.
• If separated from endogenous private consumption, government consumption can be treated as exogenous, so that different government spending levels can be considered within a meaningful macroeconomic framework.

Gross domestic product comes under the heading of national accounts, which is a subject in macroeconomics. Economic measurement is called econometrics.