An increased interest in using equilibrium thermodynamics as an independent macrotheory can be observed in recent years. From a fundamental standpoint, thermodynamics gives an universal macrodescription of nature in which using specific micromodels of objects is unnecessary. From a pragmatic standpoint, there is obviously a demand for using thermodynamics both to describe the behavior of relatively small objects (nanoparticles, etc.) at low temperatures and to study high-energy physics (including the quark–gluon plasma). As is well known, phenomenological thermodynamics is based on four laws. Among them, the zero law is basic. It relates the fundamental idea of thermal equilibrium of an object to its environment, called a heat bath. In this theory, in which all macroparameters are exactly defined, the zero law is a strict condition determining the concept of temperature: