What is Backlist?

Legal Definition
A backlist is a list of older books available from a publisher, as opposed to titles newly published (sometimes known as the frontlist).

Building a strong backlist has traditionally been considered the best method to produce a profitable publishing house, as the most expensive aspects of the publishing process have already been paid for and the only remaining expenses are reproduction costs and author royalty.

"The backlist is the financial backbone of the book industry, accounting for 25 to 30 percent of the average publisher's sales," printed The New York Times. "Current titles, known as the front list, are often a gamble: they can become best sellers, but they are much more likely to disappear in a flood of returns from bookstores. By contrast, backlist books usually have predictable sales and revenues."
-- Wikipedia
Legal Definition
The titles a publisher has that are not on the current list but hidden in the back of a catalog.