Gluten is a general name for specific proteins found in wheat, rye, and barley. It is what gives bread dough elasticity and holds it together.
Gluten proteins are made up of two types of peptides, glutenin and prolamins. It is the prolamins that trigger an autoimmune reaction in people with celiac disease. They also trigger reactions in people with gluten intolerance.
The prolamins that create a problem for celiacs are gliadin (wheat), secalin (rye), and hordein (barley). A small percentage of people with celiac disease also react to avenin, the prolamin found in oats.
The above definition of gluten is what people are talking about when they say they are on a gluten-free diet, or that they cannot eat gluten.
However, gluten is found in other foods. Rice and corn both have a form of gluten in them. It is not a gluten that causes the reactions mentioned above, but some people do have individual allergies to those foods.
You might be wondering why wheat, rye, and barley are offensive and not other grains. A plant taxonomy chart helps clarify it.
As you can see, wheat, rye, and barley are in the same tribe—triticeae. Other grasses such as rice, corn, millet sorghum,and teff (all commonly used gluten-free grains), are in the same family, but different sub families.
Other foods such as buckwheat, quinoa, and amaranth, which are also commonly use in a gluten-free diet, are in a different subclass because they are dicots. The toxic grains are monocots.
I think it is interesting to see that buckwheat is closely related to rhubarb and quinoa to spinach.
While most bakers probably love gluten for the wonderful effects it has on their edible creations, I consider it a poison. As a person with celiac disease, gluten has the power to destroy my health, and I avoid it like the plague.