If a (
musical)
keyboard supports aftertouch, it has the ability to
process,
send, or
receive pressure information while a key is being pressed.
This allows the patch programmer to create sounds that morph according to a method of user input that is a much more intuitive than a joystick or a mod wheel.
An example of a non-electronic keyboard that is aftertouch-sensitive is the clavichord. (I wrote clavinet before. Whoops!)