MIDI channel
The low four bits of a MIDI message's status byte — sixteen addresses on a single cable.
One MIDI cable can drive sixteen instruments because every channel message carries a channel number in its status byte. A synth listens on its own channel and ignores the rest, or takes everything in omni mode. User interfaces count 1–16 while the protocol says 0–15, an off-by-one mismatch that has survived for forty years.
General MIDI made channel 10 the drum channel, and the convention is so entrenched that plenty of software still assumes it. Sixteen was enough for a long time, but MPE puts every note on its own channel so it can be bent independently, and they run out quickly. MIDI 2.0 answers with sixteen groups of sixteen channels.