D3D11 stands for Direct3D 11. The Direct3D provides an API that allows apps and games to take advantage of the computing capabilities and graphics of the computers having a Direct3D compatible GPU.

OpenGL 4.0 brought feature parity with Direct3D 11's new features—in particular, compute shaders and tessellation—and with 4.1, the Khronos Group claims that it is surpassing the functionality ...

Valve releases open source Direct3D to OpenGL translator It should aid porting Windows games to other platforms, but it won't be revolutionary.