A name–value pair, also called an attribute–value pair, key–value pair, or field–value pair, is a fundamental data representation in computing systems and applications. Designers often desire an open-ended data structure that allows for future extension without modifying existing code or data. In such situations, all or part of the data model may be expressed as a collection of 2-tuples in the form <attribute name, value> with each element being an attribute–value pair. Depending on the particular application and the implementation chosen by programmers, attribute names may or may not be unique.

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Some of the applications where information is represented as name-value pairs are:

• E-mail, in RFC 2822 headers^[1]
• Query strings, in URLs
• Optional elements in network protocols, such as IP, where they often appear as TLV (type–length–value) triples
• Bibliographic information, as in BibTeX and Dublin Core metadata
• Element attributes in SGML, HTML and XML
• Some kinds of database systems – namely a key–value database
• OpenStreetMap map data
• Windows registry entries
• Environment variables

Some computer languages implement name–value pairs, or more frequently collections of attribute–value pairs, as standard language features. Most of these implement the general model of an associative array: an unordered list of unique attributes with associated values. As a result, they are not fully general; they cannot be used, for example, to implement electronic mail headers (which are ordered and non-unique).

In some applications, a name–value pair has a value that contains a nested collection of attribute–value pairs. Some data serialization formats such as JSON support arbitrarily deep nesting.^[2] Other data representations are restricted to one level of nesting, such as INI file's section/name/value.

• Attribute (computing)
• Entity–attribute–value model
• Key–value database
• Query string