Stems

In music production, stems are sub-mixes of a more extensive mix that will recreate the full mix when played together at an equal level. The purpose is to make it easier to work with the mixing process. An orchestral mix might be stemmed into strings, brass, woodwind and percussion. A rock band might be stemmed into drums, guitars, keys and vocals. By and large, stems are groups of elements that make up the final mix, not the individual elements themselves. 

The term is short for “stereo master” and originates from the movie industry, where soundtracks are usually broken down into three main stems: dialog, music and effects (DME).