(1) ASCII control character, Start of TeXt (This is not a commonly used acronym - try doing the same to RS aka Record Separator)
(2) Structured TeXt - the markup language used e.g. when editing this Wiki (see Formatting rules).
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, *, etc). Some people from Python are trying to come up with a canonical STX. I'm tracking their work, and adapting it."
The language used in editing TIPs is also a variant of STX.
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.
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.
I've written a pretty slick implementation of a wiki-like Structured Text translator, here [L1 ] - CMcC 22Apr2005