The zeroth law of thermodynamics is called a "law" because it describes a fundamental principle of how systems interact with each other in terms of heat transfer and thermal equilibrium. Here's why it earns that designation:

* It establishes a fundamental relationship: The zeroth law states that if two systems are each in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other. This seemingly simple statement has profound implications for how we understand temperature and the transfer of heat.

* It defines temperature: The zeroth law is the basis for defining temperature. It allows us to establish a scale for measuring temperature by comparing the thermal equilibrium of different systems.

* It's a universal principle: The zeroth law applies to all systems, regardless of their composition or size. This universality makes it a fundamental law of nature.

* It underlies other laws: The zeroth law is a prerequisite for the first and second laws of thermodynamics. It provides the foundation for understanding how energy is transferred and transformed within systems.

In essence, the zeroth law establishes the concept of "thermal equilibrium" and provides the framework for measuring temperature. Its fundamental and universal nature makes it worthy of being called a law of thermodynamics.