DxDiag is a diagnostic utility distributed with the DirectX package of application programming interface (API) software and multimedia components. DxDiag stands for DirectX Diagnostics. It is a tool used to generate reported on components related to DirectX, such as drivers that handle audio and video hardware. You can run DxDiag on a system with DirectX installed by using the Run box and typing "DxDiag".
Other DirectX related glossary:
DirectX Media Objects