Polyphony
How many notes an instrument can sound at once — counted in voices on a synth, each voice being a full chain of oscillator, filter and envelope.
Early analogue synths like the Minimoog were monophonic: one key at a time. The Sequential Circuits Prophet-5 of 1978 had five complete voices and a microprocessor to hand keys out between them, which made polyphony something you could actually carry into a studio. In software, voices are just instances in a loop, and the limit moves from circuit boards to the CPU.
Press more keys than there are voices and the instrument has to decide which note dies — voice stealing. The usual victim is the oldest or quietest note, ideally with a quick fade so the theft doesn't click. Paraphonic instruments share one filter across all voices, a compromise that sounds exactly like one.