Alternative spelling: "best-seller". A bestseller is a product that is selling particularly well in its field at a certain time. The term can be used to describe any kind of product, but is most frequently used in the literary world. According to Merriam-Webster, the term dates back to 1889.
Of course, what constitutes a good sale is relative to the size of the market, and there is no specific number of items sold that has to be reached before you can call the product a bestseller. A bestselling novel in Norway will, for instance, normally have sold significantly fewer copies than a novel with mediocre sales in the US or France.
The question whether success can be measured in sales figures is, naturally, a crucial one, and one that is particularly interesting when talking about literature. "Bestseller" is not just any kind of noun, it's a sales pitch. Paperback editions wear eye-catching "bestseller" labels and bestseller lists are created, based on the (correct) assumption that a lot of people will want to buy what they know a lot of people have already bought.
The sales figures don't tell us how many books ended up sitting on the bookshelf, collecting dust on the night stand or being given to people who never bothered exchanging the gift they didn't want. Nor do they tell us whether the people who started the book, finished reading it, whether they were happy with it and whether they lent it eagerly to friends or advised their friends against it. And it certainly doesn't tell us how many people borrowed it from their local library. I would really like to see, more often, polls conducted on bestselling literary works some months after the rush to buy them has calmed down, finding out how many people actually finished the book - and liked it.
Imagine picking up a book from the bookstore labelled "Most read" or "Best loved", "Most lent to friends" or even "Library smash hit, never returned until finished". Maybe that sounds silly or mushy, but I do think it represents a more healthy - and interesting - approach to literature. It's not readers that should be concerned about sales figures.