Definition
The specific VHF and UHF radio frequencies assigned to each sector of an Air Route Traffic Control Center (ARTCC), used by pilots to communicate with the controller responsible for that sector of en route airspace.
Plain English
These are the radio channels you use to talk to the controllers handling traffic in large blocks of airspace between airports. Each block has its own frequency, and you switch frequencies as you fly from one block to the next.
Context Anchor
Pilots see Center frequencies on instrument charts, in flight planning information, and hear them in controller instructions such as, “Contact Center on 127.35.”
Derivation
"Center" here is short for Air Route Traffic Control Center. The word "center" was chosen because each ARTCC sits at the center of a large region of airspace it controls. So "center frequencies" simply means the radio frequencies belonging to that controlling facility.
Why Pilots Care
Selecting the correct frequency maintains continuous contact with the controller responsible for traffic separation in that airspace.
Grounding Statement
As the airplane moves across different areas, the pilot may be given a new Center frequency so communication stays with the correct controller.
Intuition Check
Center does not mean the middle of the airport or route; here it means the Air Route Traffic Control Center controlling that area. Frequencies are not general listening channels; they are specific radio channels for reaching the correct controller.
Example Sentence 1
As we crossed into the next sector, the controller said, "Contact Center on 124.35," and we switched to the new center frequency.
Example Sentence 2
After takeoff the controller instructed the pilot to contact center on the published frequency.