WordNet (r) 3.0 (2006):

light pen
    n 1: (computer science) a pointer that when pointed at a
         computer display senses whether or not the spot is
         illuminated [syn: light pen, electronic stylus]

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

light pen

    An early pointing device which the user pointed
   at a raster-scanned display screen.  A photocell in the
   pen detected the flying spot of the raster scan.  The position
   of the spot at that instant, obtained from the scanning
   electronics, was made available to software as (x, y)
   co-ordinates.

   (2003-12-02)