proanthocyanidins, oligomers and polymers of flavonols. Proanthocyanidins are phenolics belonging to the flavonoid group, also called condensed tannins. The term proanthocyanidin refers to the reactivity of these molecules that release red anthocyanidin pigments (i.e. anthocyanin aglycones) when heated in an acidic medium, still commonly used for their analysis.

Several classes can be distinguished by the differing nature of the anthocyanidin released. The most common proanthocyanidins are procyanidins, which are the only proanthocyanidins in grape seeds, whereas tannins of grape skins and stems consist of both procyanidins and prodelphinidins.