A type of dietary fiber for which a complete structure is not well defined in part because the lignin structure itself differs between plant species (I.e., no single structure exists) and because each lignin polymer is formed as a complex pattern of monomeric units attached to protein and to structural carbohydrates. The complexity of the chemical structure of lignin can be explained by random polymerizations involving one of four main phenoxyl radicals formed upon the action of peroxidase. (The Handbook of Dietary Fiber)

Complex polymer that forms an extensive network within the cell walls of certain plants and that confers strength and rigidity to the cell wall; one of the chief substances found in wood.

The most abundant natural aromatic organic polymer found in all vascular plants. Lignin together with cellulose and hemicellulose are the major cell wall components of the fibers of all wood and grass species. Lignin is composed of coniferyl, p-coumaryl, and sinapyl alcohols in varying ratios in different plant species. (From Merck Index, 11th ed)