What is "metadata"

n. data that describes other data, serving as an informative label.

n. data about data; "a library catalog is metadata because it describes publications"

Metadata, in the Common Language Infrastructure (CLI), refers to certain data structures embedded within the Common Intermediate Language (CIL) code that describes the high-level structure of the code. Metadata describes all classes and class members that are defined in the assembly, and the classes and class members that the current assembly will call from another assembly. The metadata for a method contains the complete description of the method, including the class (and the assembly that contains the class), the return type and all of the method parameters.

A CLI language compiler will generate the metadata and store this in the assembly containing the CIL. When the run-time executes CIL it will check to make sure that the metadata of the called method is the same as the metadata that is stored in the calling method. This ensures that a method can only be called with exactly the right number of parameters and exactly the right parameter types.

The Windows Runtime application platform, present in Windows 8 and Windows Phone 8, makes use of the CLI metadata format to describe component interfaces for code written in any of the supported programming languages. A difference in use within the Common Language Runtime is that an assembly typically does not contain any CIL instructions.

Metadata are " data that provide information about other data". Two types of metadata exist: structural metadata and descriptive metadata. Structural metadata are data about the containers of data. Descriptive metadata use individual instances of application data or the data content.

Metadata were traditionally used in the card catalogs of libraries. As information has become increasingly digital, metadata are also used to describe digital data using metadata standards specific to a particular discipline. Describing the contents and context of data or data files increases their usefulness. For example, a web page may include metadata specifying what language the page is written in, what tools were used to create it, and where to find more information about the subject; this metadata can automatically improve the reader's experience.

A principal purpose of metadata is to facilitate in the discovery of relevant information, more often classified as resource discovery. Metadata also help to organize electronic resources, provide digital identification, and support the archiving and preservation of the resource. Metadata assist in resource discovery by "allowing resources to be found by relevant criteria, identifying resources, bringing similar resources together, distinguishing dissimilar resources, and giving location information."

Metadata of telecommunication activities including Internet traffic are very widely collected by various nationalities governmental organizations. These data are used for the purposes of traffic analysis and mass surveillance.