Zeroth law of thermodynamics
From RationalWiki
The zeroth law of thermodynamics states that "if two systems are in thermal equilibrium with a reservoir, then they are in thermal equilibrium with each other." It means that two systems in thermal equilibrium (i.e., they have the same amount of internal energy per unit volume) will not exchange energy.
The zeroth law is kind of a no-brainer, and was obviously known before the first, second, and third laws of thermodynamics were described. The need to formulate it as a formal law arose after the other laws had been established, so it was named the zeroth law in a backronym-ish manner.
