XInclude

XInclude is a generic mechanism for merging XML documents, by writing inclusion tags in the "main" document to automatically include other documents or parts thereof. The resulting document becomes a single composite XML Information Set. The XInclude mechanism can be used to incorporate content from either XML files or non-XML text files.

XInclude is not natively supported in Web browsers, but may be partially achieved by using some extra JavaScript code.

Example
For example, including the text file :

This document is published under GNU Free Documentation License

in an XHTML document:

gives:

The mechanism is similar to HTML's  tag (which is specific to the HTML markup language), but the XInclude mechanism works with any XML format, such as SVG and XHTML.