The Collaborative International Dictionary of English v.0.48:
Diazo- \Di*az"o-\ [Pref. di- + azo-] (Chem.)
A combining form (also used adjectively), meaning pertaining
to, or derived from, a series of compounds containing a
radical of two nitrogen atoms, united usually to an aromatic
radical; as, diazo-benzene, C6H5.N2.OH.
Note: Diazo compounds are in general unstable, but are of
great importance in recent organic chemistry. They are
obtained by a partial reduction of the salts of certain
Diazo reactions (Chem.), a series of reactions whereby
diazo compounds are employed in substitution. These
reactions are of great importance in organic chemistry.