Midlist is a term in the publishing industry which refers to books which are not bestsellers but are strong enough to economically justify their publication (and likely, further purchases of future books from the same author). The vast majority of total titles published are midlist titles, though they represent a much smaller fraction of total book sales, which are dominated by bestsellers and other very popular titles.
Authors who consistently publish acceptable but not bestselling books are referred to as Midlist authors.
Midlist publications were negatively affected by the US Supreme Court decision in the case Thor Power Tool Company v. Commissioner of Internal Revenue, which changed how book publishers have to account for and can depreciate unsold inventory each year. This has been somewhat mitigated by the development of online bookselling, which makes less popular titles more accessible to average readers. (For more on this phenomenon, see The Long Tail.)