Best-selling describes something that sells more copies than other similar items during a particular time period. A best-selling book has sold more copies than most other books recently published. A best-selling video game has attracted more buyers than its competitors.

The term usually refers to books, but can apply to any product where sales numbers matter: albums, toys, cars, or smartphones. When a bookstore displays its best-sellers, those are the books flying off the shelves fastest. The New York Times publishes a famous best-seller list every week, ranking books by how many copies were sold.

Notice that best-selling doesn't necessarily mean “best quality.” A best-selling novel might be wonderfully written, or it might just be popular because of clever marketing or a movie adaptation. Sometimes the most popular thing isn't the deepest or most skillfully made. A best-selling toy one Christmas might be forgotten by spring.

The hyphen matters here: best-selling (with a hyphen) describes the item, while best seller (two words, no hyphen) names the item itself. You'd write about “the best-selling novel of 2024” but say “that book is a best seller.”