They've always been at least "Applications" on the Mac, which is probably where the NeXTSTEP team got the file extension. On DOS/Windows it was and still is "Executable," though people are starting to casually use "App" everywhere.
Yes, Macintosh software programs were always in the 'Applications' folder. However, we never called them 'apps'. Classic MacOS didn't use file extensions, programs had a 'type code', 'APPL' (ResEdit allowed you to view those codes and edit the resources within an application). At the time, when we used a short word for software programs, we'd call them 'progs'. This changed when Apple acquired NeXT. The Yellow Box software programs in Rhapsody were the first ones I saw that had .app file extensions, and when I went to a preview of that next gen OS in 1997, Steve Jobs repeatedly called them 'apps'.
