A type of compound in which there are two rings linked through a single atom (the spiro atom). In most cases, the spiro atom is a carbon atom, and the rings are not in the same plane. Some spiro compounds with substituents on both rings may have a chirality element and consequently show optical activity.
http://www.acdlabs.com/iupac/nomenclature/79/r79_197.htm Information about IUPAC nomenclature
http://www.acdlabs.com/iupac/nomenclature/79/r79_198.htm Alternative naming system for spiro compounds