👉 In thermodynamics, a hypothetical temperature is called a "hypothetical isotherm" or a "hypothetic isothermal." This term refers to a hypothetical state of temperature where all matter in an isolated system is at equilibrium with respect to its surroundings. In this state, no heat transfer occurs between the system and its environment. The term was introduced by Karl Hermann von Helmholtz in 1879 as part of his theory on thermodynamics.
