In audio production, a stem is a discrete or grouped collection of audio sources mixed together, usually by one person, to be dealt with downstream as one unit. A single stem may be delivered in mono, stereo, or in multiple tracks for surround sound.[1]
STeM is an acronym, for “Stereo Masters”
The beginnings of the process can be found in the production of early non-silent films. In "Das Land ohne Frauen" (Land Without Women), the first entirely German-made feature-length dramatic talkie released in 1929, about one-quarter of the movie contained dialogue, which was strictly segregated from the special effects and music.