tannic acid

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A white or yellowish astringent powder, C₁₄H₁₀O₉, derived from nutgalls and used as a denaturant and in tanning and textiles.
2. n. A lustrous yellowish to light brown amorphous, powdered, flaked, or spongy mass having the approximate composition C₇₆H₅₂O₄₆, derived from the bark and fruit of many plants and used in photography, as a mordant, and to clarify wine and beer.

Wiktionary

1. n. any of various complex phenols extracted from the bark or galls of trees; used in tanning

GNU Webster's 1913

1. adj. By extension, any one of a series of astringent substances resembling tannin proper, widely diffused through the vegetable kingdom, as in oak bark, willow, catechu, tea, coffee, etc.

WordNet 3.0

1. n. any of various complex phenolic substances of plant origin; used in tanning and in medicine