It's a vitamin, mineral, herb, botanical or combination of any of those designed to add nutritional value to the diet. It's made from natural ingredients rather than synthetics.

This is a United States point of view: In the U.S., the Food and Drug Administration (FDA) regulates drugs. You can't claim that a drug cures something unless you can prove it, and you can't sell it without proving that it is both safe and effective. The Dietary Supplement Health and Education Act of 1994 established much looser rules for dietary supplements like vitamins. Supplements have to be safe, but they don't need to work. You can sell a "supplement" as long as it's harmless and as long as you only make vague claims, like "supports healthy eyesight." The word "supplement" suggests something like a vitamin, that works together with food by supplying nutrients the food might be low in. So, in the U.S. the word "supplement" means that it's legally classified as a supplement. "Natural" is a vague word with no legal meaning, but it suggests that the supplement was produced by extracting chemicals from natural sources, as opposed to synthesizing it in a factory.