IncomeIncome (revenue) is a fundamental concept in economics and accounting. One generic definition of income is "the assets received by an individual as a result of their normal business activities." Adapted to a more specific example, average American citizens' income would be the cash they receive in the form of a paycheck as a result of their employment by a company.
In economics income is the constraint to unlimited consumer purchases. Consumers can purchase a limited number of goods. The basic equation for this is I = Px*x+Py*y where Px is the price of good x, x is the quantity of good x, and I is the income (Py and y are similar to Px and x). If you need to examine more than two goods, you can add more on. This equation tells us two things. First, if you buy one more of good x, you get Px/Py less of good y. Here, Px/Py is known as the rate of substitution. Secondly, if the price of x changes, then the rate of substitution changes. This causes demand curves to slope down.
See: poverty level