In the everyday sense, variety describes the number or range of things of the same class that are distinct in some way, e.g. fruit and vegetables.
The term has a related but mathematically defined meaning in cybernetics best expressed by Ross Ashby’s Law of Requisite Variety:
If a certain quantity of disturbance is prevented by a regulator from reaching some essential variables, then that regulator must be capable of exerting at least that quantity of selection.
In other words, the variety of the regulator (i.e. the management system, the governance system) must be equal to or greater than the system it seeks to regulate. By later extension, the Law of Requisite Complexity asserts that:
The internal complexity of a system must match the external complexity it confronts.