This refers to a synthesizer that can play more than one individually articulated note at a time; in most cases, those notes all play a similar sound or patch. To pull this off, you need an oscillator, filter, amplifier, envelope, etc. per note you wish to play. That’s why most “polyphonic” synths have limited polyphony in that they can play only a certain number of notes at the same time. As a result, most modular synths are monophonic, as so much hardware and patching is required to produce just one note (although larger modular systems are often capable of playing more than one different-sounding note a time, acting as multiple monophonic synths inside one case).
There is also a clever cheat where a synth with multiple oscillators – or perhaps an organ-like “top octave divider” circuit that can produce a unique pitch for each key – can play multiple notes at once, but then mixes those oscillators down to one signal that goes through a single filter an/or amplifier. This is technically referred to a paraphonic synth; its ability to articulate individual notes is somewhat compromised.« Back to Glossary Index