From the Document: "Income is a measure of resources accruing to an individual over a period of time. In general, individuals receive income from their labor, assets, and government transfers. In its broadest terms, income is a measure of the maximum amount of goods and services an individual can consume in a given period without diminishing their net worth (the difference between their assets and liabilities) at the end of the period. Income is considered a flow variable because it is measured over a period of time; in contrast, net worth, a stock variable, is measured at a given point in time."
CRS In Focus, IF10501
Congressional Research Service: https://crsreports.congress.gov/