The ability (or restriction depending on your viewpoint) of an instrument such as a clarinet or some types of synthesiser to play only one note at a time. Generally a monophonic synthesiser will follow a rule to deal with any occasion when two notes appear. It might play the most recently received (remember that in MIDI although you think you play a chord, the notes are sent individually one after the other sufficiently fast (usually) that you think they sound together), or it might play the note with the highest pitch. Some MIDI controllers require synthesisers that can work monophonically across a number of channels, six in the case of a MIDI guitar. Thus although the synthesiser may be polyphonic it is working monophonically on each channel (2). If you think about it a real guitar can be considered to be six monophonic string instruments.
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