Modulation matrix
A table inside a synth where each row connects a modulation source to a destination by a chosen amount, replacing fixed wiring.
Sources are things that move: LFOs, envelopes, velocity, the mod wheel. Destinations are anything with a value: filter cutoff, oscillator pitch, pan, another LFO's rate. Each row says "this, there, this much" — and the amount can be negative. It is a modular synth's patch cables recast as a spreadsheet.
The EMS VCS3 of 1969 did it literally, with a pin board where small pins were pushed into a grid. Oberheim gave the idea its name with the Matrix-6 in 1985, which offered twenty rows but only a small display and buttons to edit them — powerful and famously tedious. Software synths now hide the matrix behind drag-and-drop, but the table is still there underneath.