According to the Food and Drug Administration web site: "It is difficult to define a food product that is 'natural' because the food has probably been processed and is no longer the product of the earth. That said, the FDA has not developed a definition for use of the term natural or its derivatives."
Because there's no definition, the FDA doesn't review or approve the label each time it's used. It's up to the companies to decide if their product is natural or not.
The FDA attempted to define the term "natural" in 1989, but ultimately gave up four years later, citing resource limitations and other priorities. In 1993, the FDA clarified that artificial color, flavors or synthetic substances cannot be considered natural. However high fructose corn syrup (some uses), enriched flour, and partially hydrogenated vegetable oils are considered natural by the FDA.
In the recent years there have been a spate of lawsuits against various companies for misuse of the label "natural." In these cases, the prosecutors have been individuals or groups — not the FDA.
Just this year the Kellogg Co.’s Kashi has been accused of inappropriately labeling products as natural. The class action lawsuit alleges that Kashi shakes marketed as natural contain sodium molybdate, phytonadione, and sodium selenite.