|MadSci Network: Agricultural Sciences|
The meaning of those terms is defined by the law, so the consumers won't be misleaded. In the US, those definitions are found in Part 101 Food Labeling, Title 21 Food and Drugs Volume 2 of the Code of Federal Regulation. You can check it at:
It says the following (I'll complete my answer after that):
(b) Fat content claims.(1) The terms ``fat free,'' ``free of fat,''
``no fat,'' ``zero fat,'' ``without fat,'' ``negligible source of fat,''
or ``dietarily insignificant source of fat'' or, in the case of milk
products, ``skim'' may be used on the label or in labeling of foods,
(i) The food contains less than 0.5 gram (g) of fat per reference amount customarily consumed and per labeled serving or, in the case of a meal product or main dish product, less than 0.5 g of fat per labeled serving; and
(ii) The food contains no added ingredient that is a fat or is generally understood by consumers to contain fat unless the listing of the ingredient in the ingredient statement is followed by an asterisk that refers to the statement below the list of ingredients, which states ``adds a trivial amount of fat,'' ``adds a negligible amount of fat,'' or ``adds a dietarily insignificant amount of fat;'' and
(iii) As required in Sec. 101.13(e)(2), if the food meets these conditions without the benefit of special processing, alteration, formulation, or reformulation to lower fat content, it is labeled to disclose that fat is not usually present in the food (e.g., ``broccoli, a fat free food'').
(2) The terms ``low fat,'' ``low in fat,'' ``contains a small amount of fat,'' ``low source of fat,'' or ``little fat'' may be used on the label or in labeling of foods, except meal products as defined in Sec. 101.13(l) and main dish products as defined in Sec. 101.13(m), provided that:
(i)(A) The food has a reference amount customarily consumed greater than 30 g or greater than 2 tablespoons and contains 3 g or less of fat per reference amount customarily consumed; or
(B) The food has a reference amount customarily consumed of 30 g or less or 2 tablespoons or less and contains 3 g or less of fat per reference amount customarily consumed and per 50 g of food (for dehydrated foods that must be reconstituted before typical consumption with water or a diluent containing an insignificant amount, as defined in Sec. 101.9(f)(1), of all nutrients per reference amount customarily consumed, the per 50-g criterion refers to the ``as prepared'' form); and
(ii) If the food meets these conditions without the benefit of special processing, alteration, formulation, or reformulation to lower fat content, it is labeled to clearly refer to all foods of its type and not merely to the particular brand to which the label attaches (e.g., ``frozen perch, a low fat food'').
Finally, if you want to know if a particular food is "low fat" or "fat free", you just do a chemical analysis to determine the amount of fat in it.
Try the links in the MadSci Library for more information on Agricultural Sciences.