The entropy concept was introduced in the mid-nineteenth century by Clausius and has been continually enriched, developed and interpreted by researchers in many scientific disciplines. The use of entropy in a wide range of fields has led to inconsistencies in its application and interpretation, as summarized by von Neuman “No one knows what entropy really is.” To resolve this problem, thermodynamics and other scientific disciplines face several crucial questions concerning the entropy concept: (1) What is the physical meaning of entropy? (2) Is entropy a subjective or an objective property? (3) How to apply entropy to living organisms? To answer these questions, this paper describes the roots, the conceptual history of this important concept, as well as the path of development and... application in various scientific disciplines, including classical thermodynamics, nonequilibrium thermodynamics, statistical mechanics and life sciences.