Wiktionary
n. (context publishing English) The set of unsolicited manuscripts sent to a publisher.
Wikipedia
In publishing, the slush pile is the set of unsolicited query letters or manuscripts sent either directly to the publisher or literary agent by authors, or to the publisher by an agent not known to the publisher.
Sifting through the slush pile is a job given to assistants-to-the-editors, or to outside contractors (called " publisher's readers" or "first readers"). If assistants find something interesting there and can persuade a more senior editor to consider it, they may get some credit for themselves, especially if the piece is subsequently published and sells respectably.
Most agents and the major publishing houses do not accept unsolicited manuscripts. However, smaller presses may accept queries.
In 2008 HarperCollins introduced a website, authonomy, to manage and exploit the slush pile from a web-based perspective. Website Youwriteon also acts as a slush pile filter for Random House, Orion Publishing Group and Bloomsbury Publishing.
Usage examples of "slush pile".
They'll be happy to let editors and agents sort through the slush pile, even if it means missing something like Terrapin.
And a mile north, Peter Jackson, still trying to put together the July issue of Confrontation virtually singlehanded, dives into the slush pile (which is the magazine industry's elegant name for unsolicited manuscripts) and comes up with more fallout from the Moon-Malik AUM project of 1970.
Ginjer Buchanan, who rescued this novel from the slush pile and gave me encouragement when it was needed the most.
How hard is it to get an agent, and does anybody actually read the slush pile?
What were you thinking when you picked that one out of the slush pile?
But remember, when an editor receives a script from a good agent, he gives it closer attention and more consideration than he gives to anything that comes from the slush pile.