A plain way of marking text so its structure is readable by both people and machines. A named format rather than a concept.
A twenty-first-century coinage punning on markup language: mark (Old English mearc, a sign) plus down (Old English of dune, downward); a plain way of marking text meant to be read even before it is rendered.
A named format the house writes nearly everything in — structure a person can read before a machine ever renders it.