Source : Webster's Revised Unabridged Dictionary (1913)

Polyatomic \Pol`y*a*tom"ic\, a. [Poly- + atomic.] (Chem.)
   (a) Having more than one atom in the molecule; consisting of
       several atoms.
   (b) Having a valence greater than one. [Obs.]

Source : WordNet®

polyatomic
     adj : of or relating to a molecule made up of more than two atoms