While there is no specific point that defines an author of a book as a "bestseller", becoming a bestselling author is the primary goal of many writers. In the world of romance fiction, there are several key examples of bestsellers: Nora Roberts, Jennifer Crusie, and Susan Elizabeth Phillips all come to mind.
Bestseller can also refer to the various lists published by just about every outlet from Amazon.com to the New York Times. The determination of bestseller status varies from outlet to outlet -- some rely purely on sales figures while others have a secret formula. The New York Times relies on surveys of selected bookstores, limiting the survey to specific titles. Because the NYT list does not truly reflect what is flying off of shelves in retail outlets, the have created an Extended Bestseller list to reflect real-life sales. Many romance authors, including Jo Beverley have made the New York Times Bestseller list. A book that becomes a bestseller compared to other books by the same author is considered a breakout.