What is Appropriation Bill?

Legal Definition
An appropriation bill or running bill or supply bill is a legislative motion (bill) that authorizes the government to spend money. It is a bill that sets money aside for specific spending. In most democracies, approval of the legislature is necessary for the government to spend money.

In a Westminster parliamentary system, the defeat of an appropriation bill in a parliamentary vote generally necessitates either the resignation of a government or the calling of a general election. One of the more famous examples of the defeat of a supply bill was the 1975 Australian constitutional crisis, when the Senate, which was controlled by the opposition, refused to approve a package of appropriation and loan bills, prompting Governor-General Sir John Kerr to dismiss Prime Minister Gough Whitlam and appoint Malcolm Fraser as caretaker Prime Minister until the next election (where the Fraser government was elected).
-- Wikipedia
Legal Definition
a law proposed that will authorise the spending of money and details the amount and the purpose for the expenditure.