the.com/stereo

means Sound reproduced through two or more channels to create a sense of spatial depth and direction, rather than coming from one flat point.

from From Greek 'stereos,' meaning solid or three-dimensional — the same root that gives us 'stereotype' (a solid printing plate) and 'stereoscope' (a device for viewing flat images as 3D). When audio engineers in the early-to-mid 20th century figured out how to split sound across two channels to mimic how our two ears actually locate sound in space, 'stereophonic' was the natural name — solid sound, sound with depth — later clipped down to the word we toss around now.