VXD

n. device driver that has access to the operating system kernel and can interact directly with hardware components (Computers)

VxD is the device driver model used in 16-bit versions of Microsoft Windows. They have access to the memory of the kernel and all running processes, as well as raw access to the hardware. The name "VxD" is an abbreviation for "virtual xxx driver", where "xxx" is some class of hardware device. It derives from the fact that most drivers had filenames of the form vxxxd.386 in Windows 3.x. Some examples are: vjoyd.386 (joystick), vmm.386 (memory manager). VxDs usually have the filename extension .386 under Windows 3.x and .vxd under Windows 95. VxDs written for Windows 3.x can be used under Windows 95 but not vice versa.
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Virtual Device Driver

VIRTUAL EXTENDED DRIVER [MICROSOFT]. VIRTUAL DEVICE [FILE NAME EXTENSION] [MICROSOFT]. VXD