Temperature is the thermodynamic magnitude that shows the thermal energy of one body to that of another. This thermodynamic property only describes a macroscopic state.
The definition of temperature in terms of molecular kinetic energy is the measure of the average of the molecules’ kinetic energy that makes it up. That is the movement of the particles inside it.
On the other hand, according to statistical mechanics, we can define it as the derivative of energy concerning entropy at constant volume.
Being a macroscopic magnitude, it has an exclusively statistical character. It means that it does not make sense to speak of an isolated molecule or atom’s temperature but as a whole.
The Boltzmann constant relates the average kinetic energy for each degree of freedom of an item in equilibrium to its temperature. For example, the Boltzmann constant relates the average kinetic energy of particles in an ideal gas with the gas’s temperature.