It's a horrible name that should never have been used externally, but it makes sense if you're familiar with the NT architecture. There's various environment subsystems for win32, os/2, unix, and now linux.

If you look at the graphic at the top of https://en.wikipedia.org/wiki/Architecture_of_Windows_NT it makes sense. But the fact that it needs this in order to make sense, is why it shouldn't have survived as a product name.