Forth is an imperative stack-based computer programming language and environment originally designed by Charles "Chuck" Moore. Language features include structured programming, reflection, concatenative programming (functions are composed with juxtaposition) and extensibility (the programmer can create new commands).

A procedural programming language without type checking, Forth features both interactive execution of commands (making it suitable as a shell for systems that lack a more formal operating system) and the ability to compile sequences of commands for later execution. For much of Forth's existence, the standard technique was to compile to threaded code, but there are modern implementations that generate optimized machine code like other language compilers.

Forth on Wikipedia