Pronunciation: (tan'in),[key] —n. 1. Chem.any of a group of astringent vegetable principles or compounds, chiefly complex glucosides of catechol and pyrogallol, as the reddish compound that gives the tanning properties to oak bark or the whitish compound that occurs in large quantities in nutgalls (common tannin, tannic acid). 2. any of these compounds occurring in wine and imparting an astringent taste, esp. in red wine.