A terpene, a compound found in plants. The best source of d-limonene is the oil from orange peels. Researchers at the University of Wisconsin found that when d-limonene was added to the diets of rats who had developed tumors, 90% of them had their tumors disappear completely. This has never been tested on humans, so it is not available. However, you can do what most juice experts have been saying for years: juice the orange and the peel together—just make sure your oranges are organically grown.