From the Merrimack-Webster dictionary.
Definition: a tenacious elastic protein substance especially of wheat flour that gives cohesiveness to dough.
History and Etymology for gluten: Latin gluten-, gluten glue
First Known Use of gluten: 1803, in the meaning above.
Note: The definition of gluten will give you an understanding later to the different cooking and baking process of gluten-free (wheat free) products.