A generic name for the class of simple markup languages for authoring text while preserving/indicating structure.
CMcC explained in the Tcl chatroom: "There are a lot of variants on STX, and there'll be a lot more, but they all share a couple of characteristics (e.g. significant blank lines, leading space for indenting, * for unordered list, etc). Some people from Python are trying to come up with a canonical STX. I'm tracking their work, and adapting it."
RS earlier made up the name "WikiML", which he thought might be a bit more intuitive... See Wiki format to HTML for a simple converter.
CMcC was thinking that .wml is already a well known file extension, where StructuredTeXt makes a reasonable .stx file extension, also the Python crowd seems to have a fair bit of work on the concept, and they've used StructuredText fairly widely. Certainly googling structured text will get you further than wiki markup language, and I think these markup languages are of more general strategic value than its use only on wikis.
Examples/Implementations
References/Links
http://docutils.sourceforge.net/spec/rst/introduction.html has some good thinking about the subject, courtesy of Python. I don't necessarily agree with their outcomes, but they're working toward a formal definition of the language, which IMHO is a good thing. - CMcC
http://www.zope.org/Documentation/Articles/STX here's another good intro to the general concept.