Tcl is a scripting language; nobody expands the acronym but it originally stood for Tool Control Language. More info is at http://www.tcl.tk/, and you can read more about my connection with it at http://www.aq.org/~js/aqtools/.

It’s associated with the GUI toolkit Tk, so you often hear people talk about Tcl/Tk. (Tk can also be used with Perl, Python, and other languages, but it was originally developed for Tcl.)