XML or eXtensible Markup Language is a popular markup language for storing and transporting data over the internet. It is used extensively in web development, software development, and data integration. Some common synonyms for XML include 'Extensible Hypertext Markup Language', 'eXtensible Metadata Language', and 'eXtensible Business Reporting Language'. Another popular synonym for XML is 'XHTML', or eXtensible Hypertext Markup Language. XHTML is a stricter version of HTML that adheres to XML rules and is often used in mobile web development. Whatever the term used, XML is an essential tool for developers, providing a reliable and flexible means of data storage and integration.