Mid/side
Another way of describing stereo: not left and right, but what the two channels share and what sets them apart.
Mid is the sum of the channels, side the difference: M = (L + R) / 2, S = (L − R) / 2. The conversion is lossless and fully reversible. Alan Blumlein described the idea in his 1931 stereo patent, and as a microphone technique — a directional mic facing forward plus a figure-eight facing sideways — it yields a stereo image whose width can be set after the fact.
In a DAW, M/S means an EQ or compressor can treat the centre and the edges separately: lift the vocal without touching the width, or make the bass mono. The latter is not just taste. On vinyl the side signal is cut as vertical groove motion, and too much bass there can throw the needle out of the groove. FM stereo transmits sum and difference for the same reason, so mono receivers hear only mid.