Definition
Printed boxes on IFR en route charts that list the radio frequencies used to contact an Air Route Traffic Control Center (ARTCC, or 'Center') within a given sector of airspace. Each box identifies the controlling Center facility and the frequency or frequencies a pilot should use to reach the controller responsible for that area.
Plain English
Small labeled boxes on en route charts that tell you which radio frequency to use to talk to Air Traffic Control while flying through that part of the sky.
Context Anchor
Seen on IFR en route charts during instrument flight planning and while flying an instrument route, especially when changing from one air traffic control frequency to another.
Derivation
Center' here is short for Air Route Traffic Control Center, the ATC facility that handles aircraft between terminal areas. The 'boxes' are simply the boxed labels printed on the chart to make the frequency information easy to spot.
Why Pilots Care
Using the listed frequency ensures continuous contact with the center for traffic separation, routing, and clearance changes.
Intuition Check
Do not read Center as the middle of the chart or the middle of an area. Here, Center means the air traffic control facility responsible for that part of the route.
Example Sentence 1
After losing contact with the previous controller, the pilot checked the nearest center frequency box on the en route chart and called Cleveland Center on the listed frequency.
Example Sentence 2
After handoff from approach control, the crew switched to the frequency shown in the next center frequency box.