Any one of a group of complex nonuniform plant constituents that can be classified into hydrolyzable tannins (esters of a sugar, usually glucose, and one or several trihydroxybenzenecarboxylic acids) and condensed tannins (derivatives of flavonols). Tannins are used in tanning, dyeing, photography, and as clarifying agents for beer and wine. Sometimes used synonymously with tannic acid. Tannins form black stains in the presence of iron.
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tan·nin 'tan-ən n
1) any of various soluble astringent complex phenolic substances of plant origin used in tanning, dyeing, the making of ink, and in medicine as astringents and formerly in the treatment of burns
2) a substance that has a tanning effect
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tan·nin (tanґin) tannic acid.Medical dictionary. 2011.