Decay
In the product: a page steadily losing clicks or rankings over time. Unrelated to the physics sense; product first.
In SEO Madman, decay means a page that is quietly sliding - losing clicks, impressions, or position over weeks - even if no single day looks alarming. The product surfaces decaying pages so you can refresh them before the loss compounds.
This is unrelated to the physics sense of decay (a radioactive atom breaking down). We flag it here only because the word is ambiguous: in-app, "decay" always means the traffic-loss pattern above.