Any member of a group of plant pigments (polyphenols) found in many foods that are thought to help protect the body from cancer. Flavonoids are antioxidants, though in 2007 studies published out of the research at the Linus Pauling Institute showed that they are not easily absorbed by the human body and most pass through.
Flavonoids include:anthocyanin, carotenoids, catechins, chlorophyllin, and quercetin. Current thinking (as typified in a news release from Oregon State University) is that the process of expelling them from the body triggers "so-called Phase II enzymes that also help eliminate mutagens and carcinogens, and therefore may be of value in cancer prevention... Flavonoids could also induce mechanisms that help kill cancer cells and inhibit tumor invasion." ("Studies force new view on biology of flavonoids", by David Stauth, _EurekAlert!_)