Plain English Meaning
It is the small input box on your cockpit display where you type in the DME station you want to measure distance from.
Full Definition
A field on the electronic flight display (or its associated control panel) where the pilot enters the frequency or station identifier of a Distance Measuring Equipment ground station, allowing the avionics to interrogate that station and display slant-range distance to it.
Context Anchor
Seen on electronic instrument displays when checking or setting the source for a distance readout.
Derivation
DME stands for Distance Measuring Equipment, the ground-based system that returns slant-range distance to the aircraft. 'Tuning window' comes from radio practice, where 'tuning' means selecting a frequency and 'window' refers to the on-screen field where that selection is made.
Why Pilots Care
Correct station selection supplies accurate slant-range distance information used for position fixes and approach timing.
Intuition Check
“Window” does not mean a physical window in the airplane. Here it means a small labeled area on an electronic display; “tuning” means selecting the station or channel the DME will use.
Example Sentence 1
Before starting the approach, the pilot entered the identifier for the airport's DME station into the DME tuning window so the distance display would update.
Example Sentence 2
After the DME tuning window locked onto the station, the display showed 8.4 nautical miles to the waypoint.