零定律, or the Zeroth Law of Thermodynamics, establishes a fundamental principle of thermal equilibrium. It states that if two systems, A and B, are each in thermal equilibrium with a third system, C, then A and B are also in thermal equilibrium with each other. This law allows us to define temperature in a consistent way. This law is crucial for understanding temperature measurement. It implies that temperature is a measurable property that can be used to compare different systems. The Zeroth Law lays the groundwork for the development of thermometers and the study of heat transfer in physics.