An aftertouch signal, and corresponding MIDI message, which is specific to each key on the keyboard. (As opposed to channel aftertouch, which is an average value for all held keys.) Keyboards capable of generating polyphonic aftertouch are relatively rare, since it requires a more expensive mechanism than what is needed for channel aftertouch. Polyphonic aftertouch is also known as a generator of large amounts of MIDI data, which in older equipment was capable of exceeding the maximum bandwidth of a standard MIDI cable (the so-called MIDI choke), though with modern gear this is no longer the case. It is a very expressive capability, and much desired by players who have the keyboard chops to use it. There are synths, including many rackmount synths, which can receive and process polyphonic aftertouch messages even though they cannot generate them. Most software sequencer programs will allow the performer to insert polyphonic aftertouch messages into the MIDI sequence manually.
Ensoniq was known, in the late 1980s and early 1990s, for equipping most of their synth models with polyphonic aftertouch. After Ensoniq was absorbed by Creative and stopped synth production, there was a long gap where there was nothing on the market, other than a few high-end master keyboards, with poly aftertouch capability. Starting in 2010, mass-market synths and master keyboards have begun to appear again with this capability.
The Yamaha CS80 was unique in that it was an analog synth that implemented poly aftertouch even though it did not have MIDI capability. (Aftertouch signal routing was limited to internal destinations.)