A bill that gives legal authority to spend or obligate money from the Treasury. The Constitution forbids money to be drawn from the Treasury "but in Consequence of Appropriations made by Law." By congressional custom, an appropriations bill originates in the House, and it is not supposed to be considered by the full House or Senate until a related measure authorising the funding is enacted. The latter restriction is often ignored, however.

An appropriation bill grants the actual money approved by authorization bills, but not necessarily the full amount permissible under the authorization. The 1985 Gramm-Rudman-Hollings law stipulated that the House pass by June 30 the last regular appropriations bill for the fiscal year starting October 1. (There is no such deadline for the Senate.) However, appropriations often have not been completed until well after the fiscal year begins, requiring a succession of stopgap bills to continue the government's functions. In addition, much federal spending - about half of all budget authority, notably that for Social Security and interest on the federal debt - does not require annual appropriations; those programmes exist under permanent appropriations.