A quick google search for the term will bring some mixed results. From the wikipedia definition to articles on USDA, FDA regulations and various websites and brands. The term keeps being used in food labeling and marketing with a variety of definitions, most of which are vague.
“Natural foods” are often assumed to be foods that are not processed, or do not contain any food additives, or do not contain particular additives such as hormones, antibiotics, sweeteners, food colors, preservatives, or flavorings that were not originally in the food.
Natural: Food labeled "natural," according to the USDA definition, does not contain artificial ingredients or preservatives and the ingredients are only minimally processed. However, they may contain antibiotics, growth hormones, and other similar chemicals. Regulations are fairly lenient for foods labeled "natural.”