Plain English Meaning
The aircraft figures out where it is by measuring how far it is from two ground stations at the same time, and using those two distances to pinpoint its position.
Full Definition
A method of area navigation (RNAV) position fixing in which the aircraft's onboard navigation system determines its location by simultaneously measuring slant-range distances from two or more ground-based DME stations and computing the intersection of those distance arcs.
Context Anchor
Seen in instrument procedure and area navigation discussions, especially when describing what equipment can support an RNAV departure.
Derivation
The between the two identical terms is shorthand for 'using two of these at once.' The notation reflects that a single DME gives only one distance — useful, but not a . Two DMEs give two distances, and two distances cross at a , which is enough to locate the aircraft.