The Collaborative International Dictionary of English v.0.48:

Catechin \Cat"e*chin\, n. (Chem.)
   One of the tannic acids (C15H14O6), extracted from
   catechu as a white, crystalline substance; -- called also
   catechuic acid, catechinic acid, cyanidol, and
   catechuin. It is a flavonoid found generally in higher
   woody plants, and is used in dyeing and tanning.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

catechin
    n 1: a tannic acid that is extracted from black catechu as a
         white crystalline substance