tannin

Any of a group of pale yellow to light brown amorphous substances widely distributed in plants and used chiefly in tanning leather, dyeing fabric, and making ink. Their solutions are acid and have an astringent taste. They are isolated from oak bark, sumac, myrobalan (an Asian tree), and galls. Tannins give tea astringency, colour, and some flavour. Tannins are used industrially to clarify wine and beer, reduce viscosity of oil-well drilling mud, and prevent scale in boiler water; they have also had medical uses.