Here is the definition of Data VIrtualization:

Data virtualization  is an informal, descriptive, term used in the OGSA Data Services document to describe an abstract view of some data. The Data 
Service (DS) provides a formal way to, define this abstract view of data over data resource(s) and possibly other data services, and how to access and
 modify data through this abstract view. The abstract view may be an arbitrarily complex structure with no restrictions imposed by the DS 
specification. The DS provides  interfaces to, a)  define, modify and introspect the abstract view, b) access and modify the data via this abstract 
view.  This abstract view is provided by a Data Service with a formally defined set of operations to access, and potentially modify, the data as well 
as its structure.  Within the Data Services document, the use of the term "the data virtualization" is, in almost all cases, little more than short 
hand for saying "the data accessible through the data service".