What is a Universal Windows app?

[Windows Central]

Microsoft announced a lot of new features and opportunities for developers this month at their Build conference. One of those buzz terms was ?universal Windows app?, which sure sounds remarkable. However, we?ve noticed some misunderstanding on exactly what that means. For instance, the impression by many is that if someone makes a Windows Phone app, developers can just hit a button to clone it for Windows and Windows RT devices. That leads to the ?why isn?t this not a Universal app?? question each time we mention a new release.
Microsoft is certainly on a unification kick. OneDrive, Xbox One, OneNote, mixing of Windows and Windows Phone developer registration, and expressions like ?universal Windows app? all suggest a merger of technology. It?s also a well-known secret that the Windows Phone OS and RT will eventually collide (starting with ?Threshold?), leaving just two versions of ?Windows? for Microsoft.

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