A chunk of exposition that, whether or not relevant to the plot, is insufficiently integrated into the storey being told. As such, is seems to come from left field, as if a page from an encyclopaedia accidentally got shuffled in. Asimov is famous for these. A subheading, known as "I've Suffered For My Art (And Now It's Your Turn)" occurs when the author, having done masses of boring research, proves this by unloading them on the stunned reader.