polyatomic

from WordNet (r) 3.0 (2006)

polyatomic
    adj 1: of or relating to a molecule made up of more than two
           atoms

from The Collaborative International Dictionary of English v.0.48

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.]
       [1913 Webster]