Forth is a stack-based, extensible, and interactive programming language. It was developed by Charles H. Moore in the late 1960s and early 1970s. Forth is designed to be simple and efficient, often used in embedded systems and low-level programming.

The key feature of Forth is its reliance on a stack data structure. It uses a Reverse Polish Notation (RPN) syntax, where operations are written in postfix notation. For example, instead of writing "3 + 4," you would write "3 4 +." This notation allows Forth to easily manipulate data on the stack, as operations are performed on the top elements.