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.