(1) ASCII control character, Start of TeXt
(2) Structured TeXt - the markup language used e.g. when editing this Wiki (see Formatting rules).
CMcC expalined 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.