A modular synthesizer is one where the structure of the synthesizer isn't fixed. There are a variety of modules available which can be plugged into each other. Tradionally these modules were physical boxes, but they can now be emulated with software and patched with on-screen cables....
Eight-Voice
Originally owners of the Four-Voice could expand it to up to eight SEM modules. Shown
here is an example of just that. This became the first Eight-Voice.
Later Oberheim released an Eight-Voice with two keyboards, (one on top of the other:
the top one, a 4-octave(C-C), 49-note, and the bottom one, a 5-octave(C-C), 61-note)
and an onboard Digital Programmer which held 16 presets. Currently we have no
picture of this one.