Can Anyone Define "Natural Flavor"?

I really find it interesting when I read labels on food products in my pantry....(you should try it sometimes!)....items notifying us that this item or that may have been packed on a line where milk products or tree nuts, or peanuts may have been handled. And yet, on the same package, a product contains "Natural Flavor" (you mean, like Beaver Anal Juices?)!  Why the "truth in labeling" when the food producers can put in what they want, and simply call it "Natural Flavor"?  Just some Food For Thought!