A flavonoid substance derived from apples and the bark of apple trees. As for topical application to skin, research has shown that it has antioxidant ability, can improve the look of an uneven skin tone, and also serves as a penetration enhancer, which means it can, when properly formulated, help other beneficial ingredients go beyond the superficial layers of skin.
References for this information:
The FEBS Journal, August 2008, pages 3804-3814
Phytochemistry, April 2007, pages 1189-1199
Biological and Pharmaceutical Bulletin, April 2006, pages 740-745
European Journal of Pharmaceutics and Biopharmaceutics, issue 2, March 2004, pages 307-312
International Journal of Pharmaceutics, April 2003, issues 1-2, pages 109-116