A DataBase built using XML. Small XMLDataBases might be contained within a single file or as big as the portion of the WorldWideWeb that's coded in XHTML.

XSLT is to an XMLDataBase what SQL is to a RelationalDataBase. In theory XML Schemas can be used to impose and reason about the structure of XMLDataBases, but experiance tells use that most users are unwilling to check for well-formedness, let alone validity.

In many ways the move to unstructured and semi-structured XML databases is based on a reaction to high level of rigidity in PostRelationalDataBases and a return some of the aspects of HierarchicalDataBases, with the XML tree structured document in the place of the hierarchy.