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Definition
XML (Extensible Markup Language) is a markup language for encoding structured data in a human-readable and machine-readable format. XML uses custom tags to define data elements. It is widely used in enterprise systems, document formats (DOCX, SVG), and legacy web services (SOAP).
XML was developed by the W3C in 1998 as a simplified subset of SGML. Its key feature is extensibility — unlike HTML with fixed tags, XML allows you to define custom tags that describe your data semantically. This makes XML self-documenting:
XML dominated enterprise data exchange throughout the 2000s. SOAP web services, enterprise integration buses, and B2B data exchange relied heavily on XML. The format also underpins many file formats: Microsoft Office documents (.docx, .xlsx, .pptx are ZIP archives containing XML), SVG vector graphics, RSS/Atom feeds, and Android layout files.
XML's verbosity is its biggest drawback. The same data represented in JSON is typically 30-50% smaller due to XML's opening and closing tags, attributes, and namespace declarations. JSON has largely replaced XML for web APIs and new data exchange scenarios. XML remains dominant in enterprise legacy systems, document formats, and domains where its schema validation (XSD), transformation (XSLT), and query (XPath) capabilities are valued.