- Any of a class of organic compounds that have a five-membered heterocyclic ring with at least one nitrogen and two double bonds and that also may contain another heteroatom.
- A general name given to a class of carbon-compounds containing a five-membered heteroatomic ring in which at least one atom is nitrogen, as <em>pyrazole. oxazole, thiazole</em> (which see).
- Any of a large class of compounds characterized by a five-membered ring which contains an atom of nitrogen and at least one other noncarbon atom (nitrogen, oxygen, sulphur). The prefixes <ex>furo-</ex>, <ex>thio</ex>, and <ex>pyrro-</ex> are used to distinguish three subclasses of azoles, which may be regarded as derived respectively from furfuran, thiophene, and pyrrol by replacement of the CH group by nitrogen. Names exactly analogous to those for the azines are also used
- Any of a class of five-membered unsaturated <xref>heterocycles</xref> having three <xref>carbon</xref> atoms, one <xref>nitrogen</xref> atom and two <xref>double bonds</xref>