Data about Data - don't you hate that?
Often just referring to summary information about a document (author, publishdate, etc.) or site (admin contact, date updated). But sometimes more structured information about elements within a record/document. See RDF, Dublin Core. Medscape Meta Tags gives a very detailed example.
Since it's an abstraction of data, it's a Model - 2003-10-24-MetadataModel.
Also refers to "envelope" data about a Message: see Social Graph, Messaging Security, PRISM.
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