It’s often interesting to see people’s sound-bite definitions of XML. The following, from Section 2.2 of the Static Validation of XSLT Master’s thesis by Mads Kristian Østerby Olesen, isn’t as forthright as Richard Gabriel’s but is interesting for assuming that you know about trees (in the computer sense) even if you don’t know XML:
XML is just a linear syntax for unranked, ordered and labeled trees.