Breed standard
The written description a registry holds for a breed — proportions, coat, movement, temperament — and the document judges score against in the ring.
A standard is a description of an ideal that no living animal fully matches. It fixes what the breed is supposed to be for and what it should look like doing it, and because judging awards the dogs closest to it, the standard steers what gets bred over decades.
That is also where the trouble comes from. Where a standard rewards a feature without a limit — a flatter face, a longer back, more skin — selection follows, and the health consequences arrive a generation later. Several kennel clubs have rewritten standards for exactly this reason, adding wording that a dog must be able to breathe, move and see.