A stack of unsolicited manuscripts that have arrived at an editor or publisher's office. These manuscripts will usually be read - unless the editor or publisher specifically states they will not read unsolicited works - but with less speed, interest, or enthusiasm than works submitted on spec or other request.
Stacks of manuscripts that are received by editors and publishing houses, but not specifically requested. Manuscripts in the slush pile inevitably are read only after material that an editor has sought or received by an agent. Over the years, more and more publishers have elected not to accept unsolicited manuscripts at all. Others, (typically smaller publishing houses) still keep up with slush piles, hoping to find new talent.
Manuscript submissions that are received by publishing houses, but are unsolicited or do not come through agents. Some publishers do not consider unsolicited manuscripts, while others do review them.