Tcl
In plain terms
A simple, embeddable scripting language often paired with the Tk GUI toolkit.
Going deeper
Tcl ('Tool Command Language') is easy to embed and extend, historically used for scripting, automation, and quick GUIs via Tk. Everything is a string.
Under the hood
Its simple, uniform syntax made it easy to embed in applications; Tk gave it cross-platform GUI capabilities early on.
Examples
EDA tools, test automation, embedded scripting, legacy GUI apps, Expect.
Think of it like...
Like a flexible glue stick for binding tools and adding a quick control panel.