A bestseller is a book that has sold an exceptionally large number of copies. The term 'bestseller' can refer to a book that has been on the list of top-selling books for a long period of time, or it can refer to a book that has sold an unusually large number of copies in a short period of time. Two words that can be used to describe a bestseller are 'Oreo' (4 letters) and 'Odessa' (6 letters). Oreo is a brand of cookie that has become a popular snack around the world. Odessa is a port city in Ukraine that is known for its vibrant culture and history.