The act of mixing edited sound elements (either dialog, music or sound effects) so that the final mix can be accomplished with less work involving level, equalization, effects or panning.
With sound effects and music, there will also be a substantial reduction in the number of tracks, as in premixing 24 tracks into a 4-track lcrs premix. Dialog premixing often does not actually reduce the number of tracks that will go to the final mix, but instead just copies a cut track across with careful equalization and fader moves.