What is the definition or description of: gluten?

Wheat, barley, rye: Wheat barley rye contains gluten, a protein molecule that in susceptible people can cause reactions and long term illness. There is celiac, the most well known and severe gluten disease. There is gluten sensitivity, affecting more people, but usually milder. There is wheat allergy, less common than the others. The treatment, for now, is to avoid all gluten in food, supplements, drinks, etc.