In my mind an app is a sandboxed, focused thing that lives in a single window and behind a single launcher icon. application or program or any other older term

In my mind an app is a sandboxed, focused thing that lives in a single window and behind a single launcher icon. application or program or any other older term from before iOS introduced "apps" could be a collection of a half-dozen different entries in your start menu, or an application like GIMP that scatters a bunch of different windows around your screen. "App" implies a level of simplicity.