Plain English Meaning
When an airport is too remote or too quiet to need its own approach control office, the regional center handles those duties instead. The same controllers who manage high-altitude traffic also bring IFR aircraft down to the airport and send IFR departures on their way.
Full Definition
An air traffic control service provided by an Air Route Traffic Control Center (ARTCC) that handles approach and departure functions for IFR aircraft operating to or from airports located in areas where a separate approach control facility has not been established. The center controller sequences arrivals onto an and provides and routing for departing IFR aircraft until they are released to or to the destination .