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.
[1913 Webster]