Gluten is a structural protein naturally found in certain cereal grains. Although "gluten" often only refers to wheat proteins, in medical literature it refers to the combination of prolamin and glutelin proteins naturally occurring in all grains that have been proved capable of triggering an immune reaction for people with celiac disease. These include any species of wheat (such as common wheat, durum, spelt, khorasan, emmer and einkorn), barley, rye and some oat cultivars, as well as any cross hybrids of these grains (such as triticale). Gluten makes up 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 is 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 homologous 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 (zeins) and rice (rice protein), 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 0.5–13% of the general population, as well as dermatitis herpetiformis, gluten ataxia and other neurological disorders. These disorders are treated by a gluten-free diet.