The Collaborative International Dictionary of English v.0.48:

Polysynthetic \Pol`y*syn*thet"ic\
   (p[o^]l`[i^]*s[i^]n*th[e^]t"[i^]k), a. [Poly- + synthetic.]
   Characterized by polysynthesis; agglutinative.
   [1913 Webster]

   Polysynthetic twinning (Min.), repeated twinning, like that
      of the triclinic feldspar, producing fine parallel bands
      in alternately reversed positions.
      [1913 Webster]

WordNet (r) 3.0 (2006):

polysynthetic
    adj 1: forming derivative or compound words by putting together
           constituents each of which expresses a single definite
           meaning [syn: agglutinative, polysynthetic]