Reverse Polish notation (RPN)
A way of writing expressions where the operator comes after its operands — 3 4 + instead of 3 + 4 — so parentheses become unnecessary and a single stack is enough to evaluate it.
The notation mirrors the Polish notation the logician Jan Łukasiewicz introduced in the 1920s, where the operator comes first. Move it to the end and the expression becomes "postfix", and a machine can then chew through it with one stack: push numbers on, and when an operator turns up pop the most recent numbers, compute, and push the answer back. No precedence rules, no brackets.
Hewlett-Packard made the style a cult favourite through its calculators — the HP-35 in 1972 and the long-lived HP-12C — where RPN saved both keystrokes and memory. The same idea drives languages like Forth and PostScript. Fans swear by the efficiency; everyone else wonders where the equals key went.