Source : Webster's Revised Unabridged Dictionary (1913)

Triatomic \Tri`a*tom"ic\, a. [Pref. tri- + atomic.] (Chem.)
   (a) Having three atoms; -- said of certain elements or
       radicals.
   (b) Having a valence of three; trivalent; sometimes, in a
       specific sense, having three hydroxyl groups, whether
       acid or basic; thus, glycerin, glyceric acid, and
       tartronic acid are each triatomic.