A feature of many keyboards where the performer can generate a control voltage or MIDI message 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”). Generally, when performers and manufacturers use the word "aftertouch" without qualification, they mean channel aftertouch.
Channel aftertouch refers to the average amount of pressure applied to whichever keys are held down; it is independent of which key or how many keys are held. Polyphonic aftertouch is specific to each key, meaning that the MIDI message that conveys it will contain both a note number and an amount; the channel aftertouch message contains only an amount. Relatively few synths and controller keyboards implement polyphonic aftertouch because it requires a more expensive mechanism.
The MIDI standard does not specify exactly what a receiver is supposed to do with an aftertouch message; it is usually available as a control signal that can be routed to control pitch, volume, filter cutoff frequency, etc.