Source : Webster's Revised Unabridged Dictionary (1913)

Lignin \Lig"nin\ (l[i^]g"n[i^]n), n. [L. lignum wood: cf. F.
   lignine.] (Bot.)
   A substance characterizing wood cells and differing from
   cellulose in its conduct with certain chemical reagents.

   Note: Recent authors have distinguished four forms of this
         substance, naming them lignose, lignin, lignone, and
         lignireose.

Source : WordNet®

lignin
     n : a complex polymer; the chief non-carbohydrate constituent of
         wood; binds to cellulose fibers to harden and strengthen
         cell walls of plants