  We critically revisit the definition of thermal equilibrium, in its
operational formulation, provided by standard thermodynamics. We show that it
refers to experimental conditions which break the covariance of the theory at a
fundamental level and that, therefore, it cannot be applied to the case of
moving bodies. We propose an extension of this definition which is manifestly
covariant and can be applied to the study of isolated systems in special
relativity. The zeroth law of thermodynamics is, then, proven to establish an
equivalence relation among bodies which have not only the same temperature, but
also the same center of mass four-velocity.
