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RPL. Part Lisp, part FORTH. RPL is supposed to mean "Reverse Polish Lisp", an allusion that includes the postfix nature of RPN and the mystery that is the lambda variable in Lisp.) HP never says what RPL means and doesn't even use the abbreviation in the user documentation. You'll see it (undefined) in the occasional HP Systems Journal.

RPL comes in two flavors, "user" RPL which is keyed in to the calculator and the richer (but more dangerous) "system" RPL that is compiled on an external development machine and uploaded to the calculator as a binary system object.