The absolute temperature T is the temperature defined by a gas thermometer based on the law of perfect gases.
A temperature T can be defined as the variable in the equation of state of a body f(T, P,V) = 0 where P is the pressure and V the volume of the body. For a perfect gas the equation of state PV=nRT is experimentally verified whatever the gas considered for n moles. The expansion produced by absorption of heat from a source at a temperature T will be independent of the gas used to measure this temperature.
This therefore makes it possible to define a temperature standard.
Expressed in degrees kelvin, the absolute temperature coincides with the thermodynamic temperature defined using the second principle and independently of the equation of state of the body.