Flavor compounds derived from plant or animal sources. Chemically they can be identical to 'artificial' flavors.
Labeling
In the US, 'natural flavor' means the flavor comes from a natural source — not that it's healthier or simpler.
Evidence & sources
US regulations define natural flavor by its source (spice, fruit, vegetable, meat, etc.), not by its chemical makeup. Evidence: regulatory. Known: Federal regulation. Unknown: — Source: US FDA, 21 CFR 101.22 — Foods; labeling of spices, flavorings, colorings and chemical preservatives (Current revision).