Definition
REMOTE COMMUNICATIONS AIR/GROUND FACILITY
An unmanned VHF/UHF transmitter/receiver facility which is used to expand ARTCC air/ground communications coverage and to facilitate direct contact between pilots and controllers. RCAG facilities are sometimes not equipped with emergency frequencies 121.5 MHz and 243.0 MHz.
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).