The FDA defines dietary supplements in its Guidance for Industry here: http://www.fda.gov/Food/GuidanceRegulation/GuidanceDocumentsRegulatoryInformation/DietarySupplements/ucm070519.htm#1-1
Dietary supplements are defined, in part, as products (other than tobacco) intended to supplement the diet that bear or contain one or more of the following dietary ingredients:
A vitamin;
A mineral;
An herb or other botanical;
An amino acid;
A dietary substance for use by man to supplement the diet by increasing the total dietary intake; or
A concentrate, metabolite, constituent, extract or a combination of any ingredient mentioned above.
Further, dietary supplements are products intended for ingestion, are not represented for use as a conventional food or as a sole item of a meal or the diet, and are labeled as dietary supplements. The complete statutory definition is found in section 201(ff) of Federal Food, Drug, and Cosmetic Act (the act) (21 U.S.C. 321).