Forth is the programming language Charles H. Moore developed for an observatory data-acquisition system around 1970.
Moore created Forth while working on software for controlling telescopes and processing astronomical data. He designed it to be compact, interactive, and efficient on the limited computers available at the time. The name is associated with an early system identified as the “Fourth” generation of software, shortened to “Forth” because of a character limit in the operating environment.
Forth is distinctive because it uses a postfix, or reverse Polish notation, syntax and relies heavily on a data stack. Instead of writing conventional expressions such as “2 + 3,” a Forth program commonly writes “2 3 +.” Its vocabulary can also be extended by defining new words, giving the language a small core and a highly customizable structure.
Forth has been used in embedded systems, industrial control, spacecraft-related software, and hobbyist computers. It is sometimes confused with Factor, another stack-based language, but Factor was created decades later and has different syntax and tooling.