Income is the consumption and saving opportunity gained by an entity within a specific timeframe, usually measured in money, though its exact definition varies across fields like economics and law. A cornerstone in economics is the Haig-Simons definition (1938), which describes income as consumption plus the change in net worth, a concept widely used by economists. In the US tax code, income is legally defined as any wage, salary, profit, interest, rent, or other earnings in a calendar year, while discretionary income—gross income minus taxes and mandatory deductions—serves as a common measure for comparing taxpayer welfare. For firms, gross income equals revenue minus the cost of goods sold, whereas net income further subtracts expenses, depreciation, interest, and taxes. The article also explores "full income," which includes non-monetary consumption ability (like leisure), and "factor income" from labor, capital, land, and entrepreneurship; however, economist Nicholas Barr notes that money income is an unreliable proxy for full income because it ignores non-monetary utility. In consumer theory, income acts as a budget constraint, determining how much of different goods can be purchased based on their relative prices.