Complete protein
A protein containing all essential amino acids, unlike an “incomplete protein.” Since most protein-containing foods we eat are complete proteins, the complete/incomplete distinction has little meaning.
A false dichotomy
The University of Hawaiʻi at Mānoa’s Human Nutrition textbook, available in full online, contains this statement:
Foods containing only some essential amino acids are called incomplete protein sources, while foods containing all essential amino acids are called complete protein sources. (Foods that contain some of the essential amino acids are called incomplete protein sources, while those that contain all nine essential amino acids are called complete protein sources, or high-quality protein sources.) –p255
But almost all protein-containing foods we routinely eat contain every essential amino acid, differing only in their proportions (their amino acid profiles). This makes the distinction not only meaningless but potentially misleading. A paper published in 2019 says:
One common misconception is that vegetarian diets may provide insufficient amino acids. … Terms such as “complete” and “incomplete” are misleading. In developed countries, especially in vegetarian diets, various plant proteins are eaten together, and total protein intake tends to greatly exceed requirements. (It is commonly, although mistakenly, thought that the amino acid intake may be inadequate in vegetarian diets. … The terms “complete” and “incomplete” are misleading. In developed countries, plant proteins are mixed, especially in vegetarian diets, and total intake of protein tends to greatly exceed requirement.)
Moreover, contrary to the misconception that most animal proteins are complete and most plant proteins incomplete, the most representative “incomplete” protein is actually an animal protein, gelatin. Gelatin entirely lacks the essential amino acid tryptophan.
Thus, dividing proteins into complete/incomplete and claiming that animal proteins are complete and plant proteins incomplete is both a false dichotomy and unsupported.