The term “stereo” refers to “stereophonic sound”. From Wikipedia:
The term stereophonic sound or, more commonly, stereo, refers to any method of sound reproduction in which an attempt is made to create an illusion of directionality and audible perspective.
Humans prefer to know where the sound is coming from, and all good mixes can point to some directionality of sound. Ideally people should be able to diagram where each sound is coming from (like a painting). How you achieve this? Volume, panning, reverberation, compression, etc. can move the sound left or right, and forward and backward (close or distant). So, I have seen people do this (and it works well): visualize your mix!
- Knowing where each sound source should come from,
- Envisioning in your mind’s eye where each sound should be, and
- Diagramming this to get a concrete and revisable/viewable-by-others mix image.
I have included an example diagram below for one possible mix; even simple text in boxes can give you an impression of volume, panning, reverberation, etc. in a handy portrait that you can reference any time you want to review your mix. The closer to the bottom, the more present in the mix, the closer to the top, the less (like perspective drawings). The left and right alignments indicate panning. This is only one of an infinite number of possibilities, so be creative with it!