An equation concerning the entropy S of a system that derives from statistical mechanics. It states that entropy is related to the number W of distinguishable ways in which the equation S = k lnW, where k is the Boltzmann constant, can describe the system. It expresses in quantitative terms the concept that entropy is a measure of the disorder of a system. It was discovered in the late 19th century by Ludwig Boltzmann while he was studying statistical mechanics.