Wikipedia
CityGML is a common information model for the representation of sets of 3D urban objects. It defines the classes and relations for the most relevant topographic objects in cities and regional models with respect to their geometrical, topological, semantical and appearance properties. Included are generalization hierarchies between thematic classes, aggregations, relations between objects, and spatial properties. This thematic information goes beyond graphic exchange formats and makes it possible to employ virtual 3D city models for sophisticated analysis tasks in different application domains like simulations, urban data mining, facility management, and thematic inquiries.
CityGML is an open data model and an XML-based format for the storage and exchange of data. It is implemented as an GML application schema for the Geography Markup Language 3 (GML3), the extendible international standard for spatial data exchange issued by the Open Geospatial Consortium (OGC) and the ISO TC211. CityGML is intended to become an open standard and therefore can be used free of charge.
The OGC Members adopted version 1.0.0 of CityGML as an official OGC Standard in August 2008. In late 2011, the OGC Members approved version CityGML 2.0.0.