Traditionally, gluten is defined as a cohesive, elastic protein that is left behind after starch is washed away from a wheat flour dough. Only wheat is considered to have true gluten. Gluten is actually made up of many different proteins.
It is certain that there are polypeptide chains in rye and barley proteins that are similar to the ones found in wheat. Oat proteins have similar, but slightly different polypeptide chains and may or may not be harmful to celiac patients. There is scientific evidence supporting both possibilities.
When celiac patients talk about "gluten-free" or a "gluten-free diet," they are actually talking about food or a diet free of the harmful peptides from wheat, rye, barley, and (possibly) oats. This means eliminating virtually all foods made from these grains (e. g., food starch when it is prepared from wheat, and malt when it comes from barley) regardless of whether these foods contain gluten in the very strict sense. Thus, "gluten-free" has become shorthand for "foods that dont harm celiacs."
In recent years, especially among non-celiacs, the term gluten has been stretched to include corn proteins (corn gluten) and there is a glutinous rice, although in the latter case, glutinous refers to the stickiness of the rice rather than to its containing gluten. As far as we know, neither corn nor glutinous rice cause any harm to celiacs.