This is the GNU'ish implementation of a Forth programming environment.
Forth, as a language, is best known for being stack-based, and completely extensible. Each Forth environment provides one or more dictionaries of pre-defined words, and programming in Forth consists of defining and executing new words that are combinations of previously defined words. It has been said that learning Forth changes forever the way you think about writing programs.
For more information about Forth, visit the Forth Interest Group web site at http://www.forth.org/fig.html.