tannin

noun

One of a group of complex organic chemicals, phenol derivatives, glucosides, commonly found in leaves, unripe fruits, galls of many plants and the bark of trees. Their function is uncertain, possibly giving protection or concerned with pigment formation, though the unpleasant taste may discourage grazing animals. Some tannins have commercial uses, notably as mordants for many dyes, in the production of leather (tanning) and ink. Syn. tannic acid.