From Science Europe Data Glossary
Definition: metadata is the data that describes an item such as a data set.
Explanation: Having metadata associated with a data set enables it to be found and cited. It provides other researchers with the information they require to understand the data.
An example of a metadata schema, or element set, is the widely used Dublin Core metadata schema. Examples of metadata elements are title, contributor, creator, subject, description, type, format, date, relation, and identifier.
References: http://www.dataflow.ox.ac.uk/index.php/about/about-datastage; http://www.libraries.psu.edu/psul/pubcur/what_is_dm.html#data-management; https://groups.niso.org/apps/group_public/download.php/17446/Understanding%20Metadata.pdf; https://www.w3.org/2013/dwbp/wiki/Guidance_on_the_Provision_of_Metadata;
Further details: As metadata can have different functions, distinction is often made in types of metadata, such as: descriptive, administrative and structural metadata, which can in turn be further subdivided. Another distinction that is sometimes made is between intrinsic and extrinsic metadata. Intrinsic metadata refers to the scope and semantics of the data itself (irrespective of the data format or mechanism through which it is accessed). Extrinsic metadata is concerned with describing its availability (which may be specific to particular formats and access mechanisms).