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Tannins are a diverse group of naturally occurring polyphenolic compounds found in many plants. They are complex biomolecules that are characterized by their ability to bind to and precipitate proteins and other organic compounds. This property is what gives them their name, derived from the Latin word "tannum," meaning oak bark, which was historically used for tanning leather.

Chemically, tannins are classified into two main groups: hydrolyzable tannins and condensed tannins. Hydrolyzable tannins, such as gallotannins and ellagitannins, can be broken down by hydrolysis into sugars and phenolic acids. Condensed tannins, also known as proanthocyanidins, are polymers of flavan-3-ol units and are more resistant to hydrolysis.

Tannins play a significant role in the plant kingdom. They are often found in the bark, wood, leaves, fruits, and roots of various plant species. Their presence can contribute to plant defense mechanisms against herbivores and pathogens due to their bitter taste and astringency, which can deter consumption. They also contribute to the color of certain plant parts, such as the browning of fruits as they ripen or the coloration of autumn leaves.

In human consumption, tannins are well-known for their presence in beverages like tea, coffee, wine, and beer, as well as in foods such as chocolate, nuts, and legumes. They are responsible for the astringent sensation, a dry, puckering feeling in the mouth. While generally considered safe in dietary amounts, high concentrations of tannins can interfere with nutrient absorption, particularly iron. Research also continues into their potential health benefits, including antioxidant and anti-inflammatory properties.