It states that the average kinetic energy of molecules in a gas is equal to three-halves times Boltzmann's constant times the temperature.  
Its equation result fails to predict the specific heats of polyatomic gases because the increase in internal energy associated with their heating adds energy to rotational and perhaps vibrational degrees of freedom.  However, it serves well in the definition of kinetic temperature since it involves just translational degrees of freedom.  It states that the average kinetic energy of molecules in a gas is equal to three-halves times Boltzmann's constant times the temperature.  FTP, identify this principle that relates molecules in thermal equilibrium to their average kinetic energy.
Its equation result fails to predict the specific heats of polyatomic gases because the increase in internal energy associated with their heating adds energy to rotational and perhaps vibrational degrees of freedom.  
equipartition principle