The RPL programming language (RPL meaning Reverse Polish LISP or, alternatively, ROM-based procedural language) is a handheld calculator system and application programming language used on Hewlett-Packard's engineering graphing calculators of the HP-28, HP-48 and HP-49 series.
In its internal low- to medium-level variant, called System RPL, it is used on some earlier HP calculators as well as the aforementioned ones, as part of their operating system implementation language (not accessible to the calculator user for customization and programming). The high-level User RPL version of the language is available on said graphing calculators for developing textual as well as graphical application programs. RPL is a structured programming language based on RPN but equally capable of processing algebraic expressions and formulae.
RPL has many similarities to FORTH, both languages being stack-based, and of course the list-based LISP. Contrary to previous HP RPN calculators, which had a fixed four-level stack, the RPL stack is only limited by available calculator memory.