I have heard people jest that XML stood for Excellent Marketing Language and I often felt that, unfortunately, this had become a very accurate definition. Nevertheless, the official meaning of XML is Extensible Markup Language, which remains slightly more accurate.

XML is extensible in the sense that it lets you define your own sets of elements and attributes which can be used to express virtually any hierarchical structure. The extensibility of XML has been widely used; some would even say overused. I've long since lost count of the different sets of XML elements and attributes (let's call them XML vocabularies) used by different people for different applications. Applications need to be able to tell whether documents conform to their expectations; this need creates a need for validation tools capable of representing and testing each of these vocabularies.