Zeroth law of thermodynamics

The thermodynamic principle that systems each in thermal equilibrium with a third system are in thermal equilibrium with each other. It makes temperature a consistent property for comparing systems.

Thermal equilibrium: A state in which systems in thermal contact exchange no net heat. The law's relation is defined by this state.
Heat: Energy transferred between systems because of a temperature difference. Net heat transfer ceases when systems reach thermal equilibrium.
Ralph H. Fowler: A British mathematical physicist who introduced the name “zeroth law of thermodynamics.” Fowler named the principle after the first three laws were already established.
International Temperature Scale of 1990: An internationally adopted practical temperature scale designed to approximate thermodynamic temperature reproducibly. Its fixed points and instruments operationalize temperature comparisons grounded in thermal equilibrium.
First law of thermodynamics: The energy-conservation principle applied to thermodynamic systems, relating internal-energy change to heat and work. Unlike the zeroth law, it concerns energy accounting rather than equilibrium-based temperature.
Temperature: A physical property that determines the direction of heat transfer between systems in thermal contact. The law supports treating equal temperatures as a transitive relation.
Thermodynamic system: A specified quantity of matter or region of space studied using thermodynamic properties. The law relates the equilibrium states of such systems.
Temperature scale: A numerical system assigning temperatures to thermal states using defined reference points or physical relations. The law makes a consistent scale possible by linking equilibrium states.
Fixed point (thermometry): A reproducible temperature reference defined by a specified physical state or phase transition. Fixed points anchor calibrated measurements of equilibrium temperature.
Second law of thermodynamics: The law governing entropy and the direction of spontaneous thermodynamic processes. It constrains heat-flow direction; the zeroth law establishes temperature comparison.

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