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.
Excerpted from the WQA
Glossary of Terms, 3rd Edition