National income accounting is the formal system economists use to measure a country's total economic activity, and gross domestic product, GDP, is its headline figure: the total market value of every final good and service a country produces within a given period. What makes GDP a genuinely well-tested measurement rather than a rough estimate is that it can be calculated three entirely independent ways, by total output, by total income, or by total expenditure, and all three, done correctly, arrive at the same number, because every unit of spending in an economy is simultaneously somebody's income and the value of something actually produced.
Output, income and expenditure are three views of the exact same economic activity
The output approach sums the market value of every final good and service actually produced, carefully excluding intermediate goods already counted inside some other final product, to avoid double-counting the same value twice. The income approach instead sums every form of income earned in producing that output, wages, profits, rent and interest. The expenditure approach sums every pound actually spent on final goods and services, by households, businesses, government and net exports. These aren't three different measurements that happen to be close; they're three accounting perspectives on the exact same underlying flow of economic value, since whatever gets spent buying something becomes, in the same transaction, both the value of what was produced and somebody else's income.
This three-way consistency is what makes GDP a genuinely reliable cross-check
Because these three approaches have to agree in principle, national statistical agencies routinely calculate GDP using more than one method and check the results against each other, and any meaningful gap between them signals a real measurement problem, missing data, misclassified transactions, that needs tracking down rather than simply averaging over. This built-in cross-checking is exactly why GDP has held up as economics' standard headline measure of total output for decades: it isn't just one number pulled from one data source, but a figure with genuine internal consistency across three structurally independent ways of counting the same underlying economic activity.
What we're still unsure about
That GDP can be calculated consistently through output, income and expenditure approaches, and that this three-way agreement provides a genuine internal check on the measurement, are well established, extensively used principles of national accounting. What's more genuinely an ongoing debate is exactly how well GDP itself, however accurately measured, actually captures a country's economic wellbeing, since it excludes unpaid household labour, doesn't subtract environmental damage, and treats a rise in spending on crime or disaster recovery the same as a rise in spending on genuinely beneficial goods, and economists continue to actively debate and develop alternative or supplementary measures rather than treating GDP as a fully settled, comprehensive gauge of how well an economy is actually doing for the people living in it.
This sits inside National Income Accounting & GDP, one of eight topics in Macroeconomics, one of five domains in Economics, one of seventeen subjects the app can quiz you on.