virtual device driver

from The Free On-line Dictionary of Computing (8 July 2008)

Virtual Device Driver
VxD

   <operating system> (VxD) A {device driver} under {Windows
   3.x}/{Windows 95} running as part of the {kernel} and thus
   having access to the memory of the kernel and all running
   processes as well as raw access to the hardware.  VxD's
   usually have the {filename extension} .386 under Windows 3.x
   and .vxd under Windows 95.  VxD's written for Windows 3.x can
   be used under Windows 95 but not vice versa.

   (1997-02-27)