Income, refers to consumption opportunity gained by an entity within a specified time frame, which is generally expressed in monetary terms. However, for households and individuals, "income is the sum of all the wages, salaries, profits, interests payments, rents and other forms of earnings received... in a given period of time."Case, K. & Fair, R. (2007). Principles of Economics. Upper Saddle River, NJ: Pearson Education. p. 54. For firms, income generally refers to net-profit: what remains of revenue after expenses have been subtracted. In the field of public economics, it may refer to the accumulation of both monetary and non-monetary consumption ability, the former being used as a proxy for total income.

The International Accounting Standards Board uses this definition:

Income is increases in economic benefits during the accounting period in the form of inflows or enhancements of assets or decreases of liabilities that result in increases in equity, other than those relating to contributions from equity participants. 1 (IFRS Framework)

Meaning in economics and use in economic theory

In economics, factor income is the flow (that is, measured per unit of time) of revenue accruing to a person or nation from labor services and from ownership of land and capital.

In consumer theory 'income' is another name for the "budget constraint," an amount Y to be spent on different goods x and y in quantities x and y at prices Px and Py. The basic equation for this is

Y = Px • x + Py • y.

This equation implies two things. First buying one more unit of good x implies buying Px/Py less units of good y. So, Px/Py is the relative price of a unit of x as to the number of units given up in y. Second, if the price of x falls for a fixed Y, then its relative price falls. The usual hypothesis is that the quantity demanded of x would increase at the lower price, the law of demand. The generalization to more than two goods consists of modelling y as a composite good.

The theoretical generalization to more than one period is a multi-period wealth and income constraint. For example the same person can gain more productive skills or acquire more productive income-earning assets to earn a higher income. In the multi-period case, something might also happen to the economy beyond the control of the individual to reduce (or increase) the flow of income. Changing measured income and its relation to consumption over time might be modeled accordingly, such as in the permanent income hypothesis.

Income inequality

Income inequality refers to the extent to which income is distributed in an uneven manner. Within a society can be measured by various methods, including the Lorenz curve and the Gini coefficient. Economists generally agree that certain amounts of inequality are necessary and desirable but that excessive inequality leads to efficiency problems and social injustice.