What is Forth?

Forth is a stack-based, extensible programming language without type-checking. It is probably best known for its "reverse Polish" (postfix) arithmetic notation, familiar to users of Hewlett-Packard calculators: to add two numbers in Forth, you would type 3 5 + instead of 3+5. The fundamental program unit in Forth is the "word": a named data item, subroutine, or operator.