“The term ‘entropy’ is now widely used in social science, although its origin is in physical science. There are three main ways in which the term may be used. The first invokes the original meaning, referring to the unidirectionality of heat flow, from hot bodies to cold ones. The second meaning can be derived from the first via statistical mechanics; this meaning is concerned with measures of ‘evenness’ of ‘similarity’. The third meaning derives from information theory. The three distinct meanings are carefully described and distinguished, and their relationships to each other are discussed. The various uses of the three concepts in the social sciences are then reviewed, including some uses which confuse the different meanings of the term. Finally, modern work in thermodynamics is examined, and its implications for economic analysis are briefly assessed.”