the zeroth law of thermodynamics
The zeroth law of thermodynamics states something that sounds almost too obvious to bother writing down: if object A is in thermal equilibrium with object C, and object B is also in thermal equilibrium with C, then A and B are in thermal equilibrium with each other. In plain terms, things that are each 'the same temperature as' a third thing are the same temperature as one another.
Obvious as it seems, this law is what allows temperature to be a meaningful, consistent property and lets thermometers work. Let C be a thermometer. If the thermometer reads the same value when touched to A and to B, the zeroth law guarantees A and B are at the same temperature, even though A and B were never in contact. Without this transitivity, comparing temperatures with an instrument would be meaningless.
It is called the 'zeroth' law because it was recognized as more fundamental than the already-named first and second laws, but too late to renumber them, so it was slipped in front as zero. It is the logical foundation on which the whole concept of temperature rests.
You never hold two babies' foreheads together to compare their temperatures; you touch the same thermometer to each. The zeroth law is the silent guarantee that this comparison is valid.
Comparing everything to one thermometer works only because of the zeroth law.
It was named 'zeroth' after the first and second laws already existed, because it is logically prior to both: temperature has to be well defined before those laws can even be stated.