Tcl (pronounced tickle but there is no harm in pronouncing t c l) is a programming language created by John Ousterhout in 1988. He was working on EDA (Electronic Design Automation) tools and created the language for this kind of tools. Name of the language aligns with its purpose: T stands for Tool, C is for Command and L is for Language. Although each letter represents a word, the preferred name of the language is Tcl not TCL, only T is capital (but again, there is no harm in writing TCL).
Tcl continues to improve. It is alive, it is open source, it is a simple language. It is cross platform and it can be used on almost all operating systems including macOS, Microsoft Windows and Linux.