The definition for meat by-products by the Association of American Feed Control Officials is:
The non-rendered, clean parts, other than meat, derived from slaughtered mammals. It includes, but is not limited to, lungs, spleen, kidneys, brain, livers, blood, bone, partially defatted low temperature fatty tissue, and stomachs and intestines freed of their contents. It does not include hair, horns, teeth and hoofs. It shall be suitable for use in animal food. If it bears name descriptive of its kind, it must correspond thereto.
In many cases, by-product meals are derived from "4-D" meat sources — defined as food animals that have been rejected for human consumption because they were presented to the meat packing plant as "Dead, Dying, Diseased or Disabled." The quality of animal meat by-products also tends to be very inconsistent between batches.
Meat by-products are commonly found in pet foods including Science Diet (including their prescription diet product line), Purina (both Purina One and Purina Pro Plan), Iams and Eukanuba. Ingredients listed as "meat, beef, chicken, and/or poultry by-products" on pet food labels are not required to include actual meat, and "rendered meat" on labels can refer to any rendered mammal meat, including dogs and cats.
A new category of pet food typically marketed as holistic, wellness, organic, ultra healthy, and/or simply premium pet food often emphasizes the use of human-grade meat sources only, with no animal meat by-products.