Any of a group of water soluble, natural organic phenolic compounds that are produced by metabolism in trees and plants, and are part of the degredation-resistant fulvic acid materials formed during the decomposition of vegetation. 

Tannins occur in water in almost any location where large quantities of vegetation have decayed. Tannins can impart a faintly yellowish to brown color to water. 

Tannin molecules tend to form anions in water above pH 6 and can then be treated with anion exchange resins. Below pH 5, tannins are better treated with activated carbon.