* Apple pre-1996 never referred to apps, just Applications. Neither did anyone else.
* Sun, with Java, had Applets. I don't recall them ever being shortened to 'apps'
* When Steve Jobs returned to Apple in 1996 he brought with him the term 'App'. I remember cringing every time he said it in a Keynote.
My theory is that it's a NeXT term derived from the .app extension. But for this to be true NextStep would have needed .app bundles too (and I can't remember if it did). If not it could also be from the early days of OSX.
EDIT: It seems that NeXTSTEP did have Application Bundles as I had thought: http://en.wikipedia.org/wiki/Application_Bundle
Does anyone have any concrete evidence why this isn't the case?