A bestseller is a book or other media noted for its top selling status, with bestseller lists published by newspapers, magazines, and book store chains. Some lists are broken down into classifications and specialties (novel, nonfiction book, cookbook, etc.). An author may also be referred to as a bestseller if their work often appears in a list. Well-known bestseller lists in the U.S. are published by Publishers Weekly, USA Today, The New York Times and The Washington Post. Most of these lists track book sales from national and independent bookstores, as well as sales from major internet retailers such as Amazon.com and Barnes & Noble.

In everyday use, the term bestseller is not usually associated with a specified level of sales, and may be used very loosely indeed in publishers' publicity. Books of superior academic value tend not to be bestsellers, although there are exceptions. Lists simply give the highest-selling titles in the category over the stated period. Some books have sold many more copies than current "bestsellers", but over a long period of time.

Blockbusters for films and chart-toppers in recorded music are similar terms, although, in film and music, these measures generally are related to industry sales figures for attendance, requests, broadcast plays, or units sold.

Particularly in the case of novels, a large budget and a chain of literary agents, editors, publishers, reviewers, retailers, librarians, and marketing efforts are involved in "making" bestsellers, that is, trying to increase sales.

Steinberg defined a bestseller as a book for which demand, within a short time of that book's initial publication, vastly exceeds what is then considered to be big sales.