The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   Note: Recent authors have distinguished four forms of this
         substance, naming them lignose, lignin, lignone, and
         lignireose.
         [1913 Webster]

WordNet (r) 3.0 (2006):

lignin
    n 1: a complex polymer; the chief constituent of wood other than
         carbohydrates; binds to cellulose fibers to harden and
         strengthen cell walls of plants