Requisite Variety
Etymology
From W. Ross Ashby’s Law of Requisite Variety in his 1956 "Introduction to Cybernetics." "Requisite" (Latin "requirere," to need) joins "variety," his technical term for the number of distinct states a system can take. His slogan was that only variety can destroy variety.
noun phrase · /REK-wih-zit vuh-RY-uh-tee/
The principle that to control a system, a controller must have at least as many possible responses as the system has possible states.
What it names
A rigorous handle for why oversimplified controls fail: only variety can absorb variety, so a regulator, manager, or immune system needs a repertoire matching the complexity it faces.
Example
A support script with three canned answers can’t handle a thousand kinds of complaint — it lacks requisite variety.
Related idea
Related to redundancy but not the same: redundancy is spare capacity of the same kind, while requisite variety demands enough different responses to match a varied world.