Official Aviation Definition
Distance Measuring Equipment (DME) is an internationally standardized transponder-based radio navigation system. By measuring the propagation delay of VHF/UHF radio signals between the aircraft interrogator and the ground beacon, it calculates the line-of-sight slant range distance in nautical miles.
In Simple Words
A radio system that tells the pilot exactly how many nautical miles they are away from a ground navigation station.
Real-World Flight Example
The instrument approach plate instructed the pilot to begin descent once reaching 12 DME from the airport VOR station.