A protein network formed by glutenin and gliadin in wheat (and some other grains) when flour meets water -- it's what gives bread its structure and chew.
Gluten forms almost instantly when flour is hydrated. Kneading and folding align and strengthen the network, making the dough more elastic and able to trap CO2. During fermentation, protease enzymes and bacterial proteolysis gradually break down gluten, making the dough more extensible. This breakdown is essential for a light, airy crumb -- but too much means the dough can't hold gas anymore. About 80% of wheat flour's protein is gluten.
