The Collaborative International Dictionary of English v.0.48:

Azo- \Az"o-\ [See Azote.] (Chem.)
   A combining form of azote;
   (a) Applied loosely to compounds having nitrogen variously
       combined, as in cyanides, nitrates, etc.
   (b) Now especially applied to compounds containing a two atom
       nitrogen group uniting two hydrocarbon radicals, as in
       azobenzene, azobenzoic, etc. These compounds furnish many
       artificial dyes. See Diazo-.
       [1913 Webster]