gallic acid

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A colorless crystalline compound, C₇H₆O₅, derived from tannin and used as a tanning agent, as an ink dye, and in photography and the manufacture of paper.

Wiktionary

1. n. chemistry a phenolic carboxylic acid, 3,4,5-trihydroxybenzoic acid, found in galls, tea, the bark of some trees etc; a constituent of tannin
2. n. chemistry the supposed acid H3GaO3, the acid form of amphoteric gallium hydroxide Ga(OH)3, known only as alkali metal salts

GNU Webster's 1913

1. adj. (Chem.) an organic acid, very widely distributed in the vegetable kingdom, being found in the free state in galls, tea, etc., and produced artificially. It is a white, crystalline substance, C6H2(HO)3.CO2H, with an astringent taste, and is a strong reducing agent, as employed in photography. It is usually prepared from tannin, and both give a dark color with iron salts, forming tannate and gallate of iron, which are the essential ingredients of common black ink.

WordNet 3.0

1. n. a colorless crystalline acid obtained from tannin