Definition
An unmanned VHF or UHF radio transmitter/receiver site placed at a remote location and linked back to a controlling Flight Service Station (FSS), allowing pilots to communicate with that FSS by radio from areas where direct contact would otherwise be out of range. RCOs extend the communications coverage of an FSS without requiring an additional staffed facility.
Plain English
A small, unstaffed radio site in the field that lets you talk to a Flight Service Station that is too far away to reach directly. You transmit on the RCO frequency and your voice is relayed to the FSS specialist, who answers back through the same site.
Context Anchor
Pilots commonly see RCOs listed with frequencies on charts, in airport information, and when looking for a way to contact Flight Service from the air or on the ground.
Derivation
"Remote" because the equipment sits away from the controlling station, and "outlet" because it acts as an extension point for that station's communications — like a power outlet that delivers electricity from a distant source.
Why Pilots Care
Extends communication range so pilots can obtain weather briefings, file flight plans, or receive ATC services without diverting to a manned facility.
Intuition Check
Do not picture an RCO as a staffed radio office at the airport. It is radio equipment at a remote site, controlled by people somewhere else.
Example Sentence 1
Unable to reach Flight Service directly, the pilot tuned the published RCO frequency and opened her VFR flight plan from cruise altitude.
Example Sentence 2
Because direct radio contact was unavailable, the pilot relayed the flight plan through the nearest RCO.