Quercetin is a natural plant substance that belongs to the group of flavonoids, which in turn are part of the polyphenols. It is found in various foods, especially vegetables, fruits, leafy greens and grains. Quercetin occurs in different forms, such as quercetin-3-rutinoside and quercetin-4'-glucoside.

Quercetin is commonly found in foods such as onions, apples, berries, leafy vegetables, tea and red wine. It has notable antioxidant properties and is therefore often added to dietary supplements to reap its health benefits.

The substance is named after the Latin word "quercus", meaning oak, because it was originally isolated from oak bark. Today, however, quercetin is extracted from various plant sources and is valued for its potential positive health effects, such as anti-inflammatory and immune supporting properties.