Definitions:

1. A fundamental principle that defines thermal equilibrium and forms the basis for the concept of temperature. NOUN
   • The zeroth law of thermodynamics explains why thermometers work by comparing thermal equilibrium.
2. The law that if two bodies are in thermal equilibrium with a third body then the first two bodies are in thermal equilibrium with each other. NOUN
   • The Zeroth Law is a fundamental principle in thermodynamics that has been experimentally verified to high precision.