This is the ability for parts of a synthesizer to be able to modulate other parts of the synthesizer. The synths with most cross-modulation abilities are modular synths, which actually use patch leads to plug different modules into each other, so any output can be plugged into any input....
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."