1. any of group of astringent, acidic materials derived from gallic acid. They occur in the leaves, vascular tissues, bark, unripe fruits, galls, and seed coats of many higher plants. They are found in vacuoles, cytoplasm, and sometimes cell walls or in specially enlarged tannin cells or systems of cells that appear as granular masses or yellow, red, or brown bodies. They are used to tan animal hides to produce leather, in photography to prepare the inks and dyes, and as clarifying agents for beer and wine. 2. tannin acid. 3. any of a number of substances that contain tannin. See also gallic acid, oak poisoning, acorn poisoning.