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vegetable substance capable of converting animal hide to leather, 1802, from French tannin (1798), from tan "crushed oak bark containing tannin" (see tan (v.)). Tannic acid first recorded 1836, from French acide tannique, inroduced 1834 by Pelouze.
Any of various compounds, including tannic acid, that occur naturally in the bark and fruit of various plants, especially the nutgalls, certain oaks, and sumac. Tannins are polyphenols, and form yellowish to light brown amorphous masses that can be powdery, flaky, or spongy. They are used in photography, dyeing, in tanning leather, in clarifying wine and beer, and as an astringent in medicine. Tannins are also an important ingredient in tea.