An Acronym for Tool Command Language.
It is a scripting language embedded in many applications, much like Elisp is used in Emacs and VisualBasic is in many MicrosoftWindows applications. (This does not mean these languages have anything in common; actually they're about as far apart from each other as imaginably possible.)
Tcl is included with most if not all Unix-like platforms. Its common combination with the Tk ToolKit makes creating GUIs fairly easy.
It has also a standalone interpreter like Perl, Python and Ruby but isn't as popular. Nevertheless, those other interpreters include a modified form of the Tk ToolKit library for graphics programming.
See also:
CategoryProgrammingLanguages, CategoryVeryHighLevelProgrammingLanguages
7 pages link to TCL: