ARTCC

Definition from the FAA Acronyms website:

AIR ROUTE TRAFFIC CONTROL CENTER (ARTCC)- A facility established to provide air traffic control service to aircraft operating on 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 VFR aircraft.

There are 20 such centers across the country, with 1 for Alaska and another for Hawaii. Most links found here point to the RadioReference database; if you find more up to date information, please consider posting it here, as well as supplying the information to the database team.