XML is a standard created by the W3C. It is a language with many simularities to HTML. What XML adds is the ability to define custom tags, such as , and define the meaning of those tags within the XML document itself. Thus the term "extensible." You can extend the XML language easily. XML will become more and more common as more browsers and Web servers support the XML standard.
Extensible Mark-up Language; a meta-language containing a set of rules for construction of other mark-up languages. With XML, people can make up their own tags, which expands the amount and kinds of information that can be provided about the data held in the document. XML enables designers to create their own customised tags to provide functions not available with HTML.
eXtensible Mark-up Language. A subset of SGML and a W3C standard. Widely accepted as a replacement for traditional EDI 1 for exchanging data between applications over the Internet. Unlike HTML, XML is self-defining in the sense that, when you look at an XML file, most of what you see are field names in XML tags followed immediately by a field value.