Literally, "data about data." Structured information describing information resources/objects for a variety of purposes. Although AACR2/MARC cataloging is formally metadata, the term is generally used in the library community for nontraditional schemes such as the Dublin Core Metadata Element Set, the VRA Core Categories, and the Encoded Archival Description (EAD). Metadata has been categorized as descriptive, structural, and administrative. Descriptive metadata facilitates indexing, discovery, identification, and selection. Structural metadata describes the internal structure of complex information resources. Administrative metadata aids in the management of resources and may include rights management metadata, preservation metadata, and technical metadata describing the physical characteristics of a resource. 

Reitz Joan M. (2004)