Definition
Plain language
Something about a system that is supposed to stay true no matter what happens to it.
As stated in the literature
A property that holds across all reachable states of a program or system; proving an invariant is often the crux of a correctness proof, and much of the difficulty in verified systems and olympiad combinatorics is finding the right one.
Also called: invariants
Why it matters: It matters because finding the property that always stays true is often the hardest and most crucial step in proving a system correct.
For example, a bank system should always keep the invariant that no account balance ever drops below zero.
Heard on the show
“No model ever states that invariant, even though renewal timing is the strongest positive correlate on the board.”Episode 245 — Fifteen Models Ran Football Clubs for Twenty Years, and Size Didn't Decide It