A markdown is a cut in the price of something. A shop marks goods down when they are old, on sale, or not selling: a 30% markdown means the price dropped. It is retail and business language; ordinary shoppers more often say sale, discount, or cheaper. In company money records, markdown can also mean writing down the value of stock you still hold. This entry is the price-cut meaning. Computing people also use Markdown (often with a capital M) for a simple way to write headings and lists — a joke on markup, the computer-language word. Completely different thing, and not this sense.