Forth is an imperative stack-based computer programming language and environment originally designed by Chuck Moore. Language features include structured programming, reflection (the ability to examine and modify program structure during execution), concatenative programming (functions are composed with juxtaposition) and extensibility (the programmer can create new commands). Although not an acronym, the language's name is sometimes spelled with all capital letters as FORTH, following the customary usage during its earlier years.

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 is used in the Open Firmware boot loader, in space applications^[1] such as the Philae spacecraft,^[2][3] and in other embedded systems which involve interaction with hardware. The bestselling 1986 computer game Starflight, from Electronic Arts, was written with a custom Forth.^[4]

The free software Gforth implementation is actively maintained, as are several commercially supported systems.