Gluten, according to FDA, is a mixture of proteins that occur naturally in wheat, rye, barely and crossbreeds of these grains.

FDA established its final rule defining the term “gluten-free” for voluntary use on food labels last year. The administration determined food packages can only bear the term if the food has no gluten-containing grain ingredients. However, a food product containing a gluten-containing ingredient can also be labeled gluten-free—if the ingredient has been processed to remove the presence of gluten to below 20 parts per million.

Foods that do not inherently have gluten such as bottled water, fruits and vegetables can also carry the label.

FDA says since there was previously no rule on the labeling, an estimated 5 percent of foods with a “gluten-free” label contained 20 ppm or more of gluten.

The rule was first proposed in 2007 and finally acted upon last year. Food companies had a year to comply with the new rule.

Now, foods that do not meet these terms, but carry a “gluten-free” label, will be deemed misbranded.

FDA says the decision to establish a definition for the term derived from the need to ensure the estimated 3 million people in the US with celiac disease are not misled and have accurate information.

Restaurants do not fall under the labeling rule, though FDA says those making claims on their menus should be consistent with the government agency’s definition.

The final rule can be read here.