Variety, in cybernetics, refers to the number of distinguishable elements of a set, e.g. the number of possible states of a finite state machine.
Concept introduced and most vigorously taken up by Ross Ashby, who developed the “law of experience” and the “law of requisite variety”.
The variety of an isolated system cannot increase.
This is because a deterministic transformation cannot increase variety. Therefore, an observer's uncertainty about the state of a system decreases (or at most remains constant) over time.
In a two-player game where D supplies disturbances and R tries to regulate those disturbances (maintaining a small variety of outcomes), a deterministic strategy for R can at best limit the variety in game outcomes to D's variety divided by R's variety. Only adding variety in R's moves can reduce the variety of outcomes.
The practical implication is that a perfect regulator (one which guarantees a single, specific outcome) must have at least as much variety as the phenomenon it is intended to regulate.