Stands for "Extensible Markup Language." (Yes, technically it should be EML). It is a web development format for defining specialized markup languages which can be used to transmit data that is formatted for a specific purpose. Get all that? Basically, XML is related to the hyper-text markup language (HTML), but it isn't really a markup language. It's more of a meta-language, which is a language used to create other specialized languages. Though XML is a rather new concept, it has already gained a lot of support from web developers. Don't be surprised if you see XML replacing HTML in the future.
XML
Meaning of XML in English
English glossary of computer and Internet terms. Английский глоссарий компьютерных и Интернет терминов. 2012