A set of rules for encoding documents and data that goes beyond HTML capacities. Whereas HTML is generally concerned with the semantic structure of documents, XML allows other information to be defined and passed such as <vehicle>, <make>, <model>, <year>, <color> for a car. It is the parent language of many XML-based languages such as RSS, Atom, and others. It gained further popularity with the emergence of Ajax as a way to send back data from web services, but has since lost ground to JSON, another data encoding format, which is seen as easier to work with.
