What is a 'bestseller'

On many books it’s written[ul][li]‘bestseller’[/li][li]’#1 bestseller"[/li][li]‘international bestseller’[/li][li]’#1 international bestseller’[/ul]Are these somehow standardized well-defined terms, or are they just maketing hype added by the publisher to any title they expect to sell more than a dozen of books by?[/li]
One would like to think that in order to sell a book as a “#1 bestseller”, it ought to have been at the top of some respectable chart (eg. NY Times bestseller list) at some moment, but considering how many books that are marketed as such, that does not seem plausible.

Normally this property of bestselling is attributed to the book, but sometimes one sees blurbs like ‘from the bestselling author of xyz’, where it’s clearly the author who’s selling best.

It is also worth noting that some dictionaries prefer ‘best-seller’, but ‘bestseller’ seems to be the dominating spelling on the web.

It’s a marketing term. People buy books if they think other people are buying books.

The definition depends on how much the publisher wants to stretch it. It could be “NY Times Bestseller,” which has the advantage of being easily verifiable and probably carries the most weight.

But there are other types of best sellers. It could be the best selling book that publisher ever put out. That can be legitimate (some good publishers never had anything on the NY Times list), or it could be the best of the three books they published.

Also, a book can sell many copies but not show up on the NY Times list, which has its prejudices. For instance, a Christian novel (like that “taken away” series) may sell millions upon millions of copies, but it’d never show up on the NYT list. So calling it a bestseller would be legit.

Ultimately, the term is whatever the publisher wants it to mean.