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....
OB-X
"Oberheim released the polyphonic and programmable OB-X in 1979. Available in four-,
six-, and eight-voice configurations, it offered 32 program memories, polyphonic
portamento, and polyphonic sample-and-hold."