Plain English Meaning
A regional air traffic control facility that watches over and directs IFR aircraft while they are cruising between airports, rather than near the airport itself. Pilots usually call it 'Center' on the radio.
Full Definition
An FAA facility that provides air traffic control service to aircraft operating on IFR flight plans within controlled airspace, principally during the en route phase of flight. Each ARTCC is responsible for a large geographic area of airspace divided into sectors, with controllers handing off aircraft from one sector to the next and from one Center to adjacent Centers as the flight progresses.