In Short

Universal software that can run on Windows 10 devices, including PCs, tablets, phone, or the Xbox One is referred to as Windows apps. Some software written for Windows 8.1 and Windows Phone 8.1 also fall into this category, as they can share upwards of 90 percent of their code.

The term is used to distinguish between apps built on the Windows universal app platform and traditional x86/x64 software, which is referred to as Windows desktop applications.

An example of a Windows desktop application would be Adobe Photoshop, which has a .exe, whereas a Windows app would be the Spartan Browser or APPX file.

Prior to the naming convention of Windows apps Microsoft referred to similar apps for Windows 8.1 as 'Metro apps' (and later 'Modern apps' after an alleged trademark claim by the German conglomerate Metro AG in 2012). However, these names referred mostly to their design aesthetic rather than platform model.

The official phrasing of Windows apps used to distinguish apps written for Windows 10 was revealed at WinHEC in 2015.