In experimental hadron collider physics, multijet background refers to background events whose only genuine feature are any number of jets and other hadronic activity. The jets are usually strongly produced and not the decay products of heavy resonances. Therefore, multijet background is also referred to as QCD background or hadronic background.
In analyses selecting objects other than jets, multijet events can appear as background due to misidentification. Because multijet events are much more common than most other processes, the multijet background can be large despite very small misidentification rates.