An omnibus spending bill is a type of bill in the United States that packages many of the smaller ordinary appropriations bills into one larger single bill that can be passed with only one vote in each house of Congress. There are twelve different ordinary appropriations bills that need to be passed each year (one for each appropriations sub-committee) to fund the federal government and avoid a government shutdown. An omnibus spending bill combines two or more of those bills into a single bill.
Regular appropriations bills are typically written, debated, and passed by the House and the Senate during the summer. However, these versions can be different, especially if different parties control each chamber. The omnibus negotiating process takes place later in the year, and involves reconciling differences in the various bills so that an identical, combined appropriations bill can pass both chambers and avert a government shutdown.