Source : Webster's Revised Unabridged Dictionary (1913)

Monatomic \Mon`a*tom"ic\, adv. [Mon- + atomic.] (Chem.)
   (a) Consisting of, or containing, one atom; as, the molecule
       of mercury is monatomic.
   (b) Having the equivalence or replacing power of an atom of
       hydrogen; univalent; as, the methyl radical is monatomic.

Source : WordNet®

monatomic
     adj : of or relating to an element consisting of a single atom;
           "helium and argon are monatomic gases" [syn: {monoatomic}]