Plain English Meaning
A regional control facility that watches over and directs IFR aircraft while they are flying between airports, after they leave the departure area and before they reach the arrival area.
Full Definition
An FAA facility that provides air traffic control service to IFR aircraft operating on flight plans within controlled airspace, primarily during the en route phase of flight. Each ARTCC is responsible for a large geographic area of airspace, divided into sectors, and handles aircraft between departure and arrival terminal areas.