The US Food and Drug Administration, in the Code of Federal Regulations, defines special dietary use of foods as the following:
Uses for supplying particular dietary needs that exist by reason of a physical, physiologic, pathologic, or other condition, including but not limited to the conditions of diseases, convalescence, pregnancy, lactation, allergic hypersensitivity to food, [and being] underweight and overweight;
Uses for supplying particular dietary needs which exist by reason of age, including but not limited to the ages of infancy and childhood;
Uses for supplementing or fortifying the ordinary or usual diet with any vitamin, mineral, or other dietary property. Any such particular use of a food is a special dietary use, regardless of whether such food also purports to be or is represented for general use.
Reference: Code of Federal Regulations, Title 21. Food and Drugs, Vol 2, Part 105. Foods for Special Dietary Use. Sec 105.3. Definitions and interpretations. Revised April 1, 1999. Washington, DC: US Government Printing Office