Aftertouch
The pressure you put on a key after it has already gone down, turned into a control value.
MIDI has two flavours. Channel pressure (status byte 0xD0) sends one value for the whole channel, usually the hardest press on any key. Polyphonic key pressure (0xA0) gives each key its own value, so one note in a chord can get vibrato while the rest stay put. The value is typically routed to filter cutoff, volume or LFO depth.
Yamaha's CS-80 of 1977 had polyphonic aftertouch well before MIDI existed, but the sensors were expensive and most keyboards settled for the channel variant. MPE solves the same problem by giving every note its own channel, which is an elegant way of not waiting for manufacturers to build poly aftertouch.