A file that contains text with interspersed descriptions, called "tags." All XML files are XML documents and vice versa. The XML document is often organized in a hierarchy with an "open" tag at the beginning of the file, a "close" tag at the end and all the text elements in between. Each text element has an open and close tag as well, and all tags begin with the less-than (<) character and end with the greater-than (>) character. See XML.