tannic acid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A hydrolyzable tannin in the form of a lustrous yellowish to light brown amorphous, powdered, flaked, or spongy substance having the approximate composition C₇₆H₅₂O₄₆, derived from certain plants and used in tanning leather, in fixing dyes to materials, and in clarifying wine and beer.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry any of various complex phenols extracted from the bark or galls of trees; used in tanning

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any of various complex phenolic substances of plant origin; used in tanning and in medicine