The rate at which thermal energy moves through a body due to a change in temperature. It is used in unsteady-state heat transfer calculations in which a body with a non-uniform temperature approaches equilibrium:

where k is the thermal conductivity, c_p is the specific heat capacity, and ρ is the density. Materials that have a high thermal diffusivity, such as metals, diffuse heat more quickly than materials with a low thermal diffusivity. When the temperature around the material changes, heat flows in or out of the material until thermal equilibrium is reached, assuming the environment around the material remains unchanged. Materials that have a high thermal diffusivity reach thermal equilibrium more quickly than materials with a low thermal diffusivity. The SI units are m² s⁻¹.