A means of generating a control signal based on how much pressure is applied to the keys of a MIDI keyboard. Most instruments that support this do not have independent pressure sensing for all keys, but rather detect the overall pressure by means of a sensing strip running beneath the keys. Aftertouch may be used to control such functions as vibrato depth, filter brightness, loudness and so on....
Axxe
The Axxe is basically a monophonic version of the Odyssey (which was
duophonic). It came with "Lessons in Electronic Music" (a 155 page textbook),
"Instant Axxe" sheets that you could lay over the front panel that showed you
patches, "Axxe blanks" (blank patch diagrams), an interface diagram (to hook it
up with other synths), and a book of 50 patches.
The ARP Axxe (like the Odyssey) is also equipped with "Proportional Pitch
Control (PPC), ARP's latest contribution to more expressive synthesizer
performance. The triple-pad, pressure-sensitive controller lets you bend notes
sharp and flat, and add vibrato, all without moving a single slider or switch."