(tannin) Lustrous powder, yellow to light-brown in color, that is found in tree bark (particularly oak), fruits, leaves, and tea; used medicinally as an astringent, commercially in tanning hides, as a dye mordant, and as a histological fixative and stain.

(tannins) Polyphenolic compounds with molecular weights of around 500-3000 daltons and containing enough hydroxyl groups (1-2 per 100 MW) for effective cross linking of other compounds (ASTRINGENTS). The two main types are HYDROLYZABLE TANNINS and CONDENSED TANNINS. Historically, the term has applied to many compounds and plant extracts able to render skin COLLAGEN impervious to degradation. The word tannin derives from the Celtic word for OAK TREE which was used for leather processing.