XML - eXtensible Markup Language. Like HTML, in which this web page is written, XML uses pairs of so-called tags to label (or mark-up) sections of text in order to specify how they will be used or shown. Unlike HTML, a user can define their own set of XML tags (known as a schema) for a particular purpose and as a result XML is being used as a kind of super-markup language in which specialised markups are defined. One example is SMIL (pronounced SMILE) which is a markup for synchronising time-based multimedia objects with each other and with more static things like text and web links.