A method of producing sound where the audio is mixed in two different channels. This is so that the human ears can detect direction that the sound is coming from. Usually it is used with music to give a fuller, more natural sound. It has two separate audio channels.
In a sound reproducing system, stereo refers to the use of two separate signal processing channels driving two separate power amplifiers, which in turn power two separate speaker systems. However, most times in sound reinforcement, a stereo mixer is employed to drive a mono (single channel) system in order to have (sub-mixes) separate instrument vs. vocal mixes of the program.