Tcl (originally from "Tool Command Language", but conventionally rendered as "Tcl" rather than "TCL"; is a scripting language.
It is commonly used for rapid prototyping, scripted applications, GUIs and testing. Tcl is used on embedded systems platforms, both in its full form and in several other small-footprint versions.
The combination of Tcl and the Tk GUI toolkit is referred to as Tcl/Tk.