Wikipedia
Hyperdata indicates data objects linked to other data objects in other places, as hypertext indicates text linked to other text in other places. Hyperdata enables formation of a web of data, evolving from the "data on the Web" that is not inter-related (or at least, not linked).
In the same way that hypertext usually refers to the World Wide Web but is a broader term, hyperdata usually refers to the Semantic Web, but may also be applied more broadly to other data-linking technologies such as microformats - including XHTML Friends Network.
A hypertext link simply indicates that a link exists between two documents or "information resources." Hyperdata links go beyond the simple fact of such connection, and express semantics about the kind of connection being made. For instance, in a document about Hillary Clinton, a hypertext link might be made from the word senator to a document about the United States Senate. In contrast, a hyperdata link from the same word to the same document might also state that Senator is one of Hillary's roles, titles, or positions (depending on the ontology being used to define this link).