File extension
.xml
Extensible Markup Language (XML) 1.0
.xml files are Extensible Markup Language documents: a self-describing, hierarchical text format with user-defined element and attribute names, used as a foundation for countless data and document formats.
Known meanings
Formats and reference material for .xml
Extensible Markup Language (XML) 1.0
Extensible Markup Language (XML) 1.0
Details
Format notes
Extensible Markup Language (XML)
XML is a W3C standard for encoding structured data and documents as human-readable text. It is a simplified, web-friendly descendant of SGML that lets authors define their own element and attribute names to model information as a tree of nested elements. Rather than being a single format, XML is a meta-format: it provides the syntax on which many specific formats are built, including SVG, XHTML, RSS/Atom, SOAP, Office Open XML, and innumerable configuration and data-exchange schemas.
An XML document has a strict notion of being “well-formed”: every element must be properly closed and nested, attribute values quoted, and there must be a single root element. Documents may declare an encoding, use namespaces to combine vocabularies without name collisions, and be validated for correctness against a DTD, XML Schema (XSD), or RELAX NG grammar.
Strictness And Tooling
Unlike HTML’s forgiving parsing, XML parsers reject malformed documents outright, which makes XML predictable for machine processing at the cost of being less tolerant of errors. A rich ecosystem supports it — XPath for addressing nodes, XSLT for transformation, and schema languages for validation — which is a large part of why XML remains common in enterprise and document-interchange settings.
Preservation And Security Notes
XML is well suited to preservation: it is open, text-based, self-describing, and can be validated against an explicit schema. Security requires care, however, because XML features can be abused: external entity references enable XXE attacks that read local files or make network requests, and nested entity expansion enables “billion laughs” denial-of-service. Parsers handling untrusted XML should disable external entities and DTD processing or otherwise constrain expansion.
Further Reading
- W3C XML 1.0 specification:
https://www.w3.org/TR/xml/