"Microsoft DirectX is a collection of application programming
interfaces 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.
DirectX, then, was the generic term for all of these
Direct-something APIs, and that term became the name of the
collection. Over the intervening years, some of these APIs have
been deprecated and replaced, so that this naming convention is no
longer absolute. In fact, the X has caught on to the point that it
has replaced Direct as the common part in the names of new DirectX
technologies, including XAct, XInput, and so forth..."