Real Dictionary

tannin
uncountable noun: Tannin is a yellow or brown chemical that is found in plants such as tea. It is used in the process of making leather and in dyeing.

tannin in American English
any of a group of naturally occurring phenolic compounds that precipitate proteins, alkaloids, and glucosides from solution, and convert hide into leather, including tannic acid: some are present in coffee and tea
noun: 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 nutgall

tannin in British English
noun: any of a class of yellowish or brownish solid compounds found in many plants and used as tanning agents, mordants, medical astringents, etc. Tannins are derivatives of gallic acid with the approximate formula C76H52 O46