The lignans are a large group of low molecular weight polyphenols found in plants, particularly seeds, whole grains, and vegetables. Wikipedia

Any of a class of phenylpropanoid (propylbenzene) type of molecules found in essentially all plants, generally dimeric or higher order, and produced by secondary metabolic pathways branching off of aromatic amino acid biosynthesis, in some cases having associated antioxidant or estrogenic (phytoestrogenic) activities; having in common with lignin the phenylpropanoid monomers, where lignin is a random oxidative polymerization of the same. Wiktionary