20. A system comprising: two or more databases; one or more computer processing cores; and one or more memories coupled to the computer processing cores and storing program instructions executable by the processing cores to implement an ontology harmonization and mediation engine configured to issue a query across said two or more databases by: maintaining a plurality of ontologies and ontology alignments and mappings associated with said databases through versioning and governance of metadata of said databases; describing each said database by a database schema; overlaying one or more database ontologies to each of said database schemas; mapping said database ontologies to a plurality of small domain ontologies; combining said plurality of small domain ontologies to create system ontologies; receiving a query targeting a first ontology, wherein the first ontology comprises a user analytic ontology specified by a user; translating the query into one or more translated queries, each translated query targeting a respective ontology different from the first ontology; for each of the translated queries: issuing the translated query to a respective said database organized according to the respective ontology of the translated query, and receiving a respective result set for the translated query, wherein the respective result set corresponds to the respective ontology of the translated query; for each of the respective result sets: translating the respective result set into a translated result set corresponding to the first ontology; aggregating the result sets into an aggregated result set corresponding to the first ontology; and returning the aggregated results set corresponding to the first ontology.
20. A system comprising: two or more databases; one or more computer processing cores; and one or more memories coupled to the computer processing cores and storing program instructions executable by the processing cores to implement an ontology harmonization and mediation engine configured to issue a query across said two or more databases by: maintaining a plurality of ontologies and ontology alignments and mappings associated with said databases through versioning and governance of metadata of said databases; describing each said database by a database schema; overlaying one or more database ontologies to each of said database schemas; mapping said database ontologies to a plurality of small domain ontologies; combining said plurality of small domain ontologies to create system ontologies; receiving a query targeting a first ontology, wherein the first ontology comprises a user analytic ontology specified by a user; translating the query into one or more translated queries, each translated query targeting a respective ontology different from the first ontology; for each of the translated queries: issuing the translated query to a respective said database organized according to the respective ontology of the translated query, and receiving a respective result set for the translated query, wherein the respective result set corresponds to the respective ontology of the translated query; for each of the respective result sets: translating the respective result set into a translated result set corresponding to the first ontology; aggregating the result sets into an aggregated result set corresponding to the first ontology; and returning the aggregated results set corresponding to the first ontology. 21. The system of claim 20 further comprising: mapping a target ontology to a plurality of source ontologies.