An organic heterocyclic compound containing a six-membered ring formed from carbon and nitrogen atoms. Pyridine is an example containing one nitrogen atom (C5H5N). Diazines have two nitrogen atoms in the ring (e.g. C4H4N2), and isomers exist depending on the relative positions of the nitrogen atoms. Triazines contain three nitrogen atoms.
http://www.acdlabs.com/iupac/nomenclature/93/r93_469.htm Information about IUPAC nomenclature