A generic term for that which is sensitive to and has a response to touch (presumably human). It is commonly used to denote a keyboard instrument that will respond to the performance dynamics of the player or the manner in which faders on a mixer might respond to the touch of your hand.
In keyboards, this minimally refers to a sensitivity to the velocity with which a key is struck. The idea is to give synthesizers and other keyboard instruments a performance feel similar to that of a piano. While touch sensitivity is usually used to control the volume of the instrument – defined by key velocity – it can also be used to change filter settings or many other parameters relating to the sound.
In a mixing automation system, it defines the manner in which a motorized fader responds to writing automation movement. When a finger touches a fader that is “touch sensitive” the fader responds to the electrical energy found in the finger by becoming active and ready to write automation – or fader movement.