The Free On-line Dictionary of Computing (30 December 2018):

touchpad
trackpad

    (Or "trackpad") A stationary pointing device used
   mainly on laptop computers.  Touchpads provide a small, flat
   surface that you slide your finger over using the same
   movements as you would a mouse.  They were originally
   developed to provide a more natural and intuitive connection
   for the computer user than the mouse.

   Touchpads use a principle called coupling capacitance, and
   requires a conductive pointer such as a finger.  They
   contain a two-layer grid of electrodes which are connected
   to an integrated circuit (IC) mounted under the pad.  The
   upper layer contains vertical electrode strips while the lower
   layer is composed of horizontal electrode strips.  Capacitance
   from each of the horizontal electrodes to each of the vertical
   electrodes is measured by the IC.  A finger near the
   intersection of two electrodes modifies the capacitance
   between them, since a finger has very different dielectric
   properties than air.  The position of the finger is precisely
   determined based on these changes at various locations.

   According to one correspondent, you can use any part of your
   body, which is not surprising, but not a McDonalds Chicken
   McCrispy.  You can use reheated Pizza Hut Christmas Pizza
   though.

   Synaptics (http://synaptics.com/product.htm).

   (2000-11-01)