A simplified definition to me would be a mix that doesn't have anything in it that "tugs at your ear" - Frequency spikes, strange panning and phase problems, audible noise gates, etc.
The mix should be transparent of the technology used to create it. That sounds corny I guess, but that's the best way I can explain it.
In theatre, the mood of the best performance can be quickly ruined by seeing someone's foot behind a wing back stage. It "tugs at your eyes" so to speak.
A well-balanced mix, to me, is free of the reminders of reality. ALTHOUGH, as with many things, a truly superior and inspired performance can rise above almost anything, including a poor mix.