The U.S. Federal Aviation Administration (FAA) defines an air route traffic control center (ARTCC) as a facility established to provide air traffic control service to aircraft operating on instrument flight rules (IFR) flight plans within controlled airspace and principally during the en route phase of flight. When equipment capabilities and controller workload permit, certain advisory/assistance services may be provided to visual flight rules (VFR) aircraft. There are 20 ARTCCs in the continental United States.
Outside of the United States, these facilities are often referred to as area control centres or en route centres. For example, the UK NATS' London Area Control Centre manages en route traffic in the London Flight Information Region (FIR), which includes en route airspace over England and Wales up the Scottish border.