Where the term comes from
The term describes a stretch of exposition that has grown too long. Wikipedia defines an information dump, now more commonly called an infodump, as a large drop of information by the author to provide background they think is needed to continue the plot. It calls the technique ill-advised in narrative and worse in dialogue, notes that it can slow the plot or break immersion, and says exposition usually works best when the author gives only the minimum and lets readers discover the rest as they go.
TV Tropes calls an infodump a type of exposition that is particularly long or wordy. It grants that infodumps can be done in a way that is unintrusive or entertaining, but says most of the time they are obvious and intrusive because they do not advance the plot. The alternative technique is sometimes called incluing, which Wikipedia attributes to Jo Walton, who defined it as scattering information through the text as opposed to stopping the story to impart it.
An example
Sorrel Quint's first draft of a fantasy novel opens with three pages on the history of the Candlewrights' Guild, its founding charter and its feud with the Dyers. In the revision, the heroine, Maud Teller, is late for her guild inspection with a stain on her sleeve. The inspector reminds her that Dyers' pigments are forbidden in Candlewright halls, and the reader learns the rule through the cost of breaking it. The feud appears a chapter later, when a dyer refuses to sell her thread. The history is still in the book, but it arrives one consequence at a time.
Common mistakes
- Starting the book with a lecture on setting because the writer is excited by the world.
- Having characters tell each other things they both already know, just so the reader can listen in.
- Writing the explanation in a sudden formal tone that does not match the narrator.
- Delivering rules before the reader has any reason to want them.
- Cutting all exposition and leaving the reader lost. The aim is to place it, not to remove it.
How to check your manuscript
Highlight every paragraph longer than a page that contains no action, decision or conflict. For each, ask what the viewpoint character wants in that moment and whether the information could arrive as an obstacle, a mistake or a question from someone who does not know. Pay particular attention to the first chapter, where a reader has the least patience. For nearby terms, see pacing, worldbuilding and show, don't tell.
In Musael, world and characters keeps your lore in entries beside the draft, so you can leave most of it there and let only what a scene needs reach the page.