XML
XML (Extensible Markup Language) is a markup language that encodes structured data using nested, human-readable tags, designed to be both machine-readable and self-describing across different systems and platforms.
Definition
XML (Extensible Markup Language) is a markup language that encodes structured data using nested, human-readable tags, designed to be both machine-readable and self-describing across different systems and platforms.
Overview
XML represents data as a tree of nested elements, each wrapped in opening and closing tags, optionally carrying attributes, similar in spirit to HTML but designed for structured data rather than page layout. Its defining feature is extensibility: unlike HTML's fixed set of tags, XML lets developers define their own tag vocabularies for a given domain, which is why it became the backbone of many enterprise data-exchange standards in the 2000s and early 2010s, including SOAP APIs, RSS/Atom feeds, and countless configuration and document formats (Microsoft Office files and SVG images are both XML under the hood). XML documents can be formally validated against a schema — a DTD or XML Schema (XSD) — that defines exactly which elements, attributes, and nesting are allowed, giving XML strong contract guarantees that made it attractive for regulated industries and cross-organization data exchange. XSLT, a companion transformation language, lets one XML document be converted into another format or into HTML for display. The main cost of that rigor is verbosity: the same data typically takes more characters and more parsing effort to represent in XML than in JSON, which is why JSON overtook XML as the default choice for REST APIs and browser-facing data exchange. XML remains dominant, however, wherever its stricter validation, namespacing, and document-oriented features (like mixed text and markup) genuinely matter, such as publishing, legal, and some financial and healthcare data standards. It is often mentioned alongside YAML in this space.
Key Features
- Self-describing, nested tag-based structure similar to HTML
- Custom, domain-specific tag vocabularies (extensibility)
- Formal schema validation via DTD or XML Schema (XSD)
- Transformable into other formats via XSLT
- Foundation for SOAP, RSS/Atom, SVG, and many Office document formats
- More verbose than JSON but stronger on document-oriented and mixed content