1. Any of a large group of water-soluble antioxidant compounds, including the anthocyanins, flavonols, flavones, and isoflavonoids, often occurring as glycosides in plants and consisting of two aromatic rings linked by a carbon bridge that often forms a heterocyclic ring. Flavonoids are found in tea, red wine, and a variety of vegetables and fruits.

2. Any of various compounds that are a subset of this group, including the flavones, flavanols, and flavonols, as distinguished from the isoflavonoids. In both senses also called bioflavonoid.