publishing language in (de)basement

What  does bestseller (or best seller or best-seller) or bestselling author mean in our trade? It seems to me that too many publishers are certainly doing their utmost to make the answer to that question, "not much" or "I'm not sure". Perhaps, as a recent reading spree with a batch of publishers catalogs and regular reading of some of the trade media would indicate, the terms are abused, misused, and debased more within the trade than in advertising to consumers. I'd hope the publishing community isn't as lax and manipulative about this terminology with the consumer we want and need to buy books, but I doubt it. Since book advertising is diminishing at a rapid rate, I'm sure the primary use and misuse of "bestseller" and "bestselling" are on the covers and flap copy of the books. 

Admittedly, I'm a quick reader with catalogs and ads (i.e. a skimmer - except when something catches my eye due either to a book being particularly interesting to me personally or professionally or - with the perverse quirks of my mind - when my eye catches a felicitous or disasterous phrase or an interesting cover or a title or when I think either "what a great idea" or "what was the publisher thinking?" (Of course, that publisher may be twice as smart as I am or know exactly what she is doing and who the customers for the book and just how to reach them. That's part of the professional fun.)

In the trade, does a "bestseller" need to have made a list...any list...anywhere? If it has achieved that distinction,  would it be an undue burden to be honest with booksellers and rights buyers about just which list or lists? Heck, a tiny footnote would do. Or , in a deeper debasement, does it simply mean a bestseller within the limited context of the publisher's own program? Or does showing  up in the lists or rankings (either the overall rankings or category rankings) of the  top (pick-a-number, or will any number do?) of a single bookseller suffice? If a list or a ranking from a single bookseller is enough to declare one's book a bestseller, does not referencing that bookseller imply success in the wider world? Indeed, is avoiding attributing the source of that particular claim to bestsellerdom merely a means of not ticking off that bookseller's competitors while still wanting to claim the distinction? Do the various "extended lists" from media that are largely invisible to consumers count?

Or, given how widely these terms are used without any context and often any "believability" factor, does it really matter at all? Do we in the book trade really, in fact, disregard these publishers descriptions of their own products unless we have particular reason to believe them or they get specific? And what of the book buyer—has the industry also inured him or her to what should have some meaning?