Q.Define thermal equilibrium between two bodies, and explain what it means for a body to be in thermal equilibrium with a thermometer.

Two bodies are in thermal equilibrium when, placed in contact through a heat-conducting (diathermic) wall, no net heat flows between them and their macroscopic properties stop changing -- this condition corresponds to equal temperature. The Zeroth law of thermodynamics states that if a system $A$ is in thermal equilibrium with a system $C$, and a system $B$ is separately also in thermal equilibrium with $C$, then $A$ and $B$ must be in thermal equilibrium with each other as well, without ever needing to be placed in direct contact. Despite being logically the most basic of the thermodynamic laws, it was named "zeroth" because it was identified as a necessary postulate only after the first and second laws were already established. Its importance is that it is exactly what justifies temperature as a single, well-defined, measurable quantity: taking the third system $C$ to be a thermometer, the law guarantees that if the thermometer reads the same value for two different bodies, those bodies are genuinely at the same temperature -- which is the logical foundation underlying every practical use of a thermometer.