Definition in literature? Impossible. I can tell you about my associations, though.
Slop literature is a continuation of what was called "littérature de la gare", then pulp (I specifically avoid this term because it has associations with a certain type of subject matter, and I am less interested in the general nature of this type of literature than in subject matter). Another, more apt name is "industrial literature", born in the industrial era and produced like an industry.
Slop is convenient, easy, simple, easy to replace. It gives quick pleasure, empty calories. It is something worse than ordinary entertainment literature, which preserves the proper features of a literary work (or at least it used to be distinguishable), by which I mean an appropriately high technical, psychological level and avoidance of infantilism.
In other words, literature assumes that a mature individual operates a craft appropriately. This is what distinguishes Chandler (a writer) from Hammett (a sloper with the makings of a writer). For example - Eisenhorn or Gorek & Felix. Both series have their moments, even some depth, but too quickly they fall into very gamey brawls, inability to lead the action in a sensible direction or plain childishness.
Just like Harlequins.
And yet Gotrek and Eisenhorn are the better ones (I can also add books by Newman and Watson, forgotten nowadays). And yet there is the edgelord Bowden and the really weak Goto and a lot of nameless slopers.