Tannin is any of a group of complex organic substances, containing phenols, hydroxy acids, or glucosides, which occur occuring widely in plants, dissolved in cell-sap. Tannins are particularly common in the bark of oak, mangrove, and sumac, unripe fruits, leaves, and oak galls, and are extracted by boiling in water. They may be classified as hydrolyzable (yielding gallic acid) or condensed. Tannins are used for tanning, for making dyes and inks, and in medicine as an astringent.