An ontology is
an explicit specification of a conceptualization
. Ontologies are seen as important technique for semantic data processing, and in particular for interoperability. As they represent knowledge about a certain evolving real world domain the ontologies have to evolve as well. Knowledge about the changes is mandatory to correctly interpret data or documents based on the semantics defined in the ontology. Furthermore, the correct comparison of data and documents from different points in time, based on different versions of an ontology is only possible if the differences between these versions are known.