Two channel audio system. Often not binaural, i.e. two distinct and unique tracks.  
For example many stereo recorders require that both tracks be recorded simultaneously. 
Most music is now produced stereophonically which refers to the spatial distribution of sound between two channels. 
A single monaural sound may be applied to both channels with different amplitudes in order to "place" it in the stereo field. If the amplitude is the same in each channel the resulting sound will appear to "float" in the middle of the two loudspeakers used to reproduce the two channels. As the amplitude of the mono signal is increased on one channel the sound will appear to have its spatial source nearer to the speaker reproducing that channel. 
In a multitrack environment the placement or movement of monaural tracks between output channels is known as panning. 
See Also:
Spatial perception     Doubling     Binaural localization