I disagree with this analysis. I think the term “killer app” came into the mainstream lexicon as a single unit. People not familiar with the term “app” could deduce its meaning out of context, and I don’t think your everyday computer user was using the term “app” casually until the media pounced upon this phrase. While I knew what “appz” were before the 90s, the phrase “killer app” to me strikes me as a bit of technical jargon that some reporters in the sector pounced on because it sounded jazzier than something like “flagship program” or “foundational software.” This then entered the mainstream press as the entire phrase.
That makes sense - thanks 
Back when Lode Runner and Dark Castle were hits at Moscone center, Aldus Pagemaker was being described as the “killer App” for microcomputers. That was in 1986 or so.
It was something we all wanted a copy of in our App Folders.
Who is “we”? I’m not saying the term wasn’t in use–as I stated, I was familiar with it in the 80s, too. I don’t recall it being mainstream at the time, and I knew it from a different context than an “App Folder.” I didn’t hear that until well into the 90s.
Those of us who went to Macworld, 1986.
That’s just “marketroid-speak”. You can’t expect marketroids to use plain english, can you, when there is a perfectly meaningless popular buzz-word available? 
Well, yes, you certainly have a point, and I will agree with it, but that just brings up what I stated in the first paragraph of the post, which you didn’t quote. The proper question to answer the OP is “What is the definition of mainstream?” When the term “killer app” began to appear isn’t a good definition of it, for the simple reason that it’s use implies that some number of people were already using “app” by itself. How many? Enough to consider mainstream? Just pointing to the first appearance doesn’t address those questions, and thus doesn’t address the OP. It’s just a way of generating an arbitrary date solely for the sake of drawing a line. That’s not my impression of what the OP was asking.
I think it is, but we have different opinions. I feel that “killer app” is where the casual computer users became familiar with the term “app.” I interpret that to be what part of what the OP is asking. The exact title: is “how/when did the word ‘app’ become so mainstream”? I (and others) posit that it first jumped from jargon or Mac- and tech-centric usages to the mainstream in the term “killer app” and then, later, into completely everyday lingo with the iPhone and its advertising campaign.
[bolding added]
Absent a better definition from the OP for “mainstream”, I will provisionally buy that argument.
[quote=“Squink, post:43, topic:550546”]
I agree. I remember the use of the term ‘app’ as part of compound terms such as ‘killer app’ and ‘app folder’, however, the use of the term ‘app’ by itself I’ve only heard used as software that is somehow subordinate, more narrow in focus, or available only on a particular mobile medium, and only really in popular use since the "there’s an app for that’ ads of Apple…
Wikipedia dates the origin of the term ‘killer App’ back with the VisiCalc spreadsheet on the Apple II.
1979 clearly predates the presence of Apps in a Mac’s ‘Application Folder’.
If you check OED2 there is a quote from a national newspaper, without any further explanation, in 2000.
Such a use would seem to indicate that it was ‘mainstream’ at that time.
Not to throw another wrench into the works, but now, upon re-reading the actual thread title, we actually aren’t working with the definition of mainstream, but rather “so mainstream.”
Yes and I think it was definitely the iPhone ads that kicked it from being a word that most people understood to a word that most people used.
This is how I parse the OP, too. “So mainstream” would be beginning with the Apple iPhone advertisement onslaught. Before that, I normally only heard the term in conjunction with “killer app” or perhaps “app folder.” For the life of me, though, I can’t remember whether we referred to the folder on Macs as the “app folder” or just “applications.” I want to say it was always “app(s) folder,” but I have a feeling I may be filtering that through my current usage. I think I probably said something like “Go to your applications” or “Go to Applications” rather than “Go to your app(s) folder,” back in the mid-to-late-90s. But I honestly have no clue what the standard usage was around my fellow Mac users.
So, you are saying that it’s the inquisitorial equivalent of a tautology.
When did an expression become as mainstream as it did after the beginning of a massive advertising campaign for a popular product by some company?
Of course, the answer to that, not surprisingly, is: after the beginning of a massive advertising campaign for same popular product by same company.
Just because a massive campaign uses a word it doesn’t necessarily follow that the campaign made the word popular, however in this case I believe it did.