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.
   [1913 Webster]

   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
         amido compounds.
         [1913 Webster]

   Diazo reactions (Chem.), a series of reactions whereby
      diazo compounds are employed in substitution. These
      reactions are of great importance in organic chemistry.
      [1913 Webster]

WordNet (r) 3.0 (2006):

diazo
    adj 1: relating to or containing diazonium