Appropriations
The annual laws that give federal agencies legal authority to spend money.
Authorising a program and funding it are two separate acts of Congress. An authorisation says a program may exist and sets its terms; an appropriation provides the budget authority to actually spend on it.
The appropriations process is handled by the Appropriations Committees in each chamber, divided into twelve subcommittees whose jurisdictions map to the twelve regular bills. Mandatory spending — Social Security, Medicare, interest on the debt — flows from permanent law and is not part of this annual process, which is why the annual fight covers a minority of total federal spending.
Twelve regular appropriations bills fund the discretionary side of the government each year. Without one — or a continuing resolution — an agency cannot obligate funds.