Stereophonic: using two or more channels of transmission
Monophonic: using only one channel of transmission
When sitting in front of a pair of monitors or wearing a pair of headphones, you may listen to a sound that is perceived from straight in front of you. However, there is no central speaker in front of you nor on the headphones you are wearing. How can this be?
This effect is the result of the left monitor and the right monitor producing a sound that has the same frequency, same amplitude and same phase at the same time. Since there is no audible difference between the two sources of sound, our brain perceives that sound from the centre of the two sources. A stereo sound can be defined as any small or extreme difference between the left and right channels pertaining to frequency, amplitude, phase or time.
Any change in amplitude, frequency, phase or time will be detected by our brains and the stereo image will shift. The type and direction of shift depends heavily on which aspect you change.