Answer Description
Microsoft DirectX is a application program interface (API) used by software developers to create images, animations, and other multimedia effects for video animations and games for Windows Operating Systems. Open Graphics Library (OpenGL) is a similar open source API that is usable on most Apple/Macintosh, Linux, Unix, and Microsoft Operating Systems.
Wikipedia
Microsoft DirectX is a collection of application programming interfaces (APIs) for handling tasks related to multimedia, especially game programming and video, on Microsoft platforms Originally, the names of these APIs all began with Direct, such as Direct3D, DirectDraw, DirectMusic, DirectPlay, DirectSound, and so forth The name DirectX was coined as a shorthand term for all of these APIs (the X standing in for the particular API names) and soon became the name of the collection When Microsoft later set out to develop a gaming console, the X was used as the basis of the name Xbox to indicate that the console was based on DirectX technology
DirectX - Wikipedia, the free encyclopedia