It's jargon, and in this case “knowledge decay” refers to how fast students forget what they have learned for a test. There's a general sense among faculty that they forget a lot, quickly. Research would respond to our general impressions with answers that clearly support “it depends” conclusions. A study done in a chemistry class illustrates this. The research team started with the big general question. Does knowledge decay occur in chemistry courses? And if it does, when? The study answered these questions with data collected in three different courses: an undergraduate chemistry course for nursing students, another for nonscience undergraduates, and another for high school honors students.