The Collaborative International Dictionary
Polyatomic
Polyatomic \Pol`y*a*tom"ic\, a. [Poly- + atomic.] (Chem.)
Having more than one atom in the molecule; consisting of several atoms.
Having a valence greater than one. [Obs.]
Wiktionary
polyatomic
a. (context chemistry English) (''of a molecule or ion'') Consisting of three or more atoms.
WordNet
polyatomic
adj. of or relating to a molecule made up of more than two atoms