A best-seller is something that a huge number of people buy. We use it most for books: a novel on a “best-seller list” is one that is selling very fast this week. Though actually, “best” here does not mean the book is the finest one ever written. It only means many people paid for it. A thin holiday cookbook or a noisy children’s story can be a best-seller. You will also see it written as one word, bestseller, or as two words, best seller; all three are used. Shops stretch it past books to anything that flies off the shelf — a perfume, a toy, a phone. It is everyday English with a shop-window feel, not a school-science word.