Due to some reason, what we referred as “Metro” in Windows 8 is not to be used anymore. Those last few months have left us in the dark, so here is some clarification:
Metro style apps = Windows Store apps (note lowercase on apps)
Metro design principles = Microsoft design style principles
Metro design language = Microsoft design language
Not sure yet for the first one, but the last two could also be applied to Windows Phone, XBOX, …
There have been several reference to “Modern UI” this summer but it was just temporary.
You can find references to these new names in this MSDN article: Make great Windows Store apps