A system for naming heterocyclic compounds, independently introduced for naming 5- and 6- membered nitrogen monocyclic compounds by A. Hantzsch (1887) and O. Widman (1888). Subsequently it was extended to other ring sizes and other heteroatoms. It is based on prefixes denoting the hetero atoms(s) (e.g. aza- for nitrogen, thia- for sulphur, oxa- for oxygen), and items denoting ring size and saturation. For example, 4- membered rings have ete for unsaturated rings and etane for saturated; 5- membered rings have ole for unsaturated and olane for saturated compounds. Rules are given for naming compounds with two or more different heteroatoms and for numbering the atoms. The system is widely used both for monocyclic compounds and for heterocyclic components of polycyclic compounds.
http://www.chem.qmul.ac.uk/iupac/hetero/HW.html Information about IUPAC nomenclature