Aftertouch

From AudioLexic

A feature of many keyboards where the performer can generate a control signal by varying the amount of force applied to the keyboard while holding down one or more notes. Aftertouch divides into two categories, channel aftertouch (referred to in the MIDI standard as “channel pressure”) and polyphonic aftertouch (referred to in the MIDI standard as “key pressure”). Both channel aftertouch and polyphonic aftertouch have their own message types in the MIDI standard.