A DataBase built using XML. Small XMLDataBases might be contained within a single file and large ones may span all of the WorldWideWeb that's coded in XHTML.
XSLT is a query language for XMLDataBases in much the same way that SQL is a query language for RelationalDataBases.
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. In theory XML Schemas can be used to reason about the structure of XMLDataBases, but experiance tells use that most users are unwilling to check for well-formedness, let alone validity.