Most consumers would probably apply the word "stereo" exclusively to 2-channel audio systems. So you might be surprised to learn that "stereo" has nothing to do with the number 2 per seit derives from the Greek word for "solid." The word was applied to 2-channel sound systems when they first became available because those systems rendered a much more "solid" sonic image with more specific placement of individual elements (instruments, voices, etc.) than the monaural systems that had preceded them. Similarly, 5.1 surround systems are far more "stereo" than 2-channel systems.