Gluten is a group of seed storage proteins found in certain cereal grains. Although, strictly speaking, "gluten" pertains only to wheat proteins, in the medical literature this term is used to refer to the combination of prolamin and glutelin proteins naturally occurring in all grains that have been demonstrated capable of triggering celiac disease. These include any species of wheat (such as common wheat, durum, spelt, khorasan, emmer and einkorn), barley, rye and oats, as well as any cross hybrids of these grains (such as triticale). Gluten comprises 75–85% of the total protein in bread wheat.Glutens, especially Triticeae glutens, have unique viscoelastic and adhesive properties, which give dough its elasticity, helping it rise and keep its shape and often leaving the final product with a chewy texture. These properties, and its relatively low cost, make gluten valuable to both food and non-food industries.Wheat gluten are composed of mainly two types of proteins the glutenins and the gliadins, which in turn can be divided into high molecular and low molecular glutenins and α/β, γ and Ω gliadins. Its homologus seed storage proteins in barley, are referred to as hordeins; in rye, secalins; and in oats, avenins. These protein classes are collectively referred to as "gluten". The storage proteins in other grains, such as maize and rice, are sometimes called gluten, but they do not cause harmful effects in people with celiac disease.

Gluten can trigger adverse inflammatory, immunological and autoimmune reactions in some people. The spectrum of gluten related disorders includes celiac disease in 1–2% of the general population, non-celiac gluten sensitivity in 6–10% of the general population, as well as dermatitis herpetiformis, gluten ataxia and other neurological disorders. These disorders are treated by a gluten-free diet.