Principle of Least Astonishment
Law of Least Surprise · POLA · POLS
The notion that a component of a system should behave in a way that most users will expect it to behave; the behavior should not astonish or surprise users.
Origin
The earliest documented use appeared in December 1967, when W. N. Holmes invoked the "Law of Least Astonishment" in the PL/I Bulletin to criticize inconsistencies in IBM's new programming language. The principle gained wider currency through the Unix philosophy in the 1970s and 1980s, and Eric S. Raymond formalized it as a core design tenet in The Art of Unix Programming (2003).
Everyday Use
When you press Ctrl+S, you expect "save" — not "send." When a door has a handle, you expect to pull it. The principle says: if your design surprises people, the design is wrong. It applies to software, physical products, and any system where a human expects one thing and gets another.