Common among the various definitions of metadata are the notions of "descriptive data" and "structure". In reality metadata exists everywhere in very common forms. Metadata is the formal term for descriptive information about a digital or physical object such as a person, place, thing, or even an activity. "Structure" refers to how this descriptive information is presented. A phone directory (contact metadata about people and organizations) and a library catalog (bibliographic metadata about documents) are two examples of structured metadata. Metadata is most often used as a tool for resource discovery - the information that can be searched to locate a digital or physical object.
Several definitions of "metadata" have been cited in literature, including:
- "...structured data about data that imposes order on a disordered information universe"
- "Data about data" or "information about data" - an integral part of a data set
- Description of the content, quality, condition, and other characteristics of data.
- Metadata assist in locating and understanding data