METADATA is information about data provided by a data supplier with includes a description of the content, format, quality, utility and origin of the data, electronic representation of the data (data is any kind of organized information that represents something relevant, usually formatted as a text, image, sound, video or any other format), security and rights information and other appropriate characteristics.

Usually metadata is used to reference different information as an index, so people can easily identify this or that information within a context.

Some definition of metadata found on the web:

WIKIPEDIA Metadata (Greek: meta- + Latin: data "information"), literally "data about data", is information that describes another set of data. A common example is a library catalog card, which contains data about the contents and location of a book: It is data about the data in the book referred to by the card.

WEBOPEDIA Data about data. Metadata describes how and when and by whom a particular set of data was collected, and how the data is formatted. Metadata is essential for understanding information stored in data warehouses and has become increasingly important in XML-based Web applications.

NASA GCMD For the purposes of the Global Change Master Directory (GCMD), metadata is defined as: "Descriptive information that characterizes a set of quantitative and/or qualitative measurements and distinguishes that set from other similar measurement sets."