[1] It is a measure of the rate of heat transfer inside a material and has SI units of m2/s.
Thermal diffusivity is usually denoted by lowercase alpha (α), but a, h, κ (kappa),[2] K,[3] ,D,
Diffusivity is the more fundamental concept and describes the stochastic process of heat spread throughout some local volume of a substance.
Effusivity describes the corresponding transient process of heat flow through some local area of interest.
In special cases of steady state, the thermal conductivity (k) may be sufficient to describe heat transfers inside solid or rigid bodies by applying Fourier's law.
[8][9] It involves heating a strip or cylindrical sample with a short energy pulse at one end and analyzing the temperature change (reduction in amplitude and phase shift of the pulse) a short distance away.