Novel protein
A protein the individual animal has never eaten before, used in food-allergy work-ups because the immune system cannot react to something it has not met.
Novel is relative to the patient, not to the market. Venison is novel for a dog raised on chicken and useless for one that has been eating venison for a year. That is the whole logic of an elimination diet: strip the diet down to one protein and one carbohydrate the animal has no history with, feed nothing else for the weeks it takes, then reintroduce foods one at a time.
Two things spoil it. Treats and flavoured medication count as food. And several studies of over-the-counter limited-ingredient diets have found proteins not named on the label, which is why vets doing a genuine work-up tend to reach for a hydrolysed or prescription diet instead.