http://arxiv.org/abs/nucl-th/0603028 Authors: D.H.E.Gross Comments: 7 pages, 1 figure, presnted at XILV internat. winter meeting on nuclear physics in Bormio(Italy) 2006, January 29- Febr. 5 Subj-class: Nuclear Theory; Statistical Mechanics The physical meaning of entropy is analyzed in the context of statistical, nuclear, atomic physics and cosmology. Only the microcanonical Boltzmann entropy leads to no contradictions in several simple, elementary and for thermodynamics important situations. The conventional canonical statistics implies several serious errors and misinterpretations. This has far reaching consequences for phase-separations as well for the usual formulations of the second law. Applications in cosmology suffer under the ubiquitous use of canonical statistics. New reformulations in terms of microcanonical statistics are highly demanded but certainly difficult.