It’s the protein found in wheat, rye, and barley.
Technically, gluten is a group of proteins in wheat, but it is now used as a catch-all term for the proteins that are toxic to people with Celiac Disease/Gluten Sensitivity. It’s actually a specific protein found within the wheat, rye, and barley that causes the problems for Celiacs/Gluten sensitive people. In wheat, the protein is called gliadin. In rye, it is secalin, and in barley, it is hordein. Gliadin, secalin and hordein are closely related and all affect Celiacs/Gluten sensitive people.