An appropriation bill is a legislative motion which authorizes the government to spend money. In most democracies, approval of the legislature is necessary for the government to spend money.

The defeat of an appropriation bill in a parliamentary vote generally necessitates either a resignation of a government or the calling of a general election.

In the United States, two types of legislation are used to authorize spending. An authorization is a piece of legislation that permits spending for "mandatory" programs. A mandatory program is one that does not need an additional piece of legislation known as an appropriation in order for spending to occur. Social Security benefits are an example of a "mandatory" program. An authorization also makes known the intent of the Congress about the level of spending for programs that also require an appropriation. In the United States, an appropriation bill is used to permit spending for "discretionary" programs. What distinguishes a mandatory from a discretionary program is that after an authorization bill is enacted, a "mandatory" program is permitted to spend funds until the program expires based on a provision in the enacted bill, or until a subsequent piece of legislation either terminates the program or reauthorizes it. "Discretionary" programs typically require annual appropriations legislation. The defeat of an appropriation bill in the United States requires the passage of another appropriation bill in order for continued spending to occur, or passage of a bill known as a continuing resolution, which generally permits spending at prior year levels.