Fahrenheit was an effort to create a unified high-level API for 3D computer graphics to unify Direct3D and OpenGL. It was designed primarily by Microsoft and SGI and also included work from an HP-Microsoft joint effort.
Direct3D and OpenGL are low-level APIs that concentrate primarily on the rendering steps of the 3D rendering pipeline. Programs that use these APIs have to supply a considerable amount of code to handle the rest of the pipeline. Fahrenheit hoped to provide a single API that would do most of this work, and then call either Direct3D or OpenGL for the last steps.
Much of the original Fahrenheit project was abandoned, and Microsoft and SGI eventually gave up on attempts to work together. In the end, only the scene graph portion of the Fahrenheit system, known as XSG, saw a release and was discontinued shortly afterwards.