Variety has always been a fundamental idea in Cybernetics and Systems Science, and is so in Metasystem Transition Theory.
Variety is defined as a multiplicity of distinctions.
The existence of variety is necessary for all change, choice, and information. The simplest is the count of the number of distinctions. More useful can be the logarithm of that number as a quantity of information, which is called the Hartley entropy. The most celebrated are the stochastic entropies of classical information theory, which result from applying probabilistic distributions to the various distinctions. A reduction in the quantity of variety is the process of selection .