A material's thermal effusivity is a measure of its ability to exchange thermal energy with its surroundings.
If two semi-infinite bodies initially at temperatures T1 and T2 are brought in perfect thermal contact, the temperature at the contact surface Tm will be given by their relative effusivities. 
This expression is valid for all times for semi-infinite bodies in perfect thermal contact. It is also a good first guess for the initial contact temperature for finite bodies.
- heat capacity
- heat equation
- laser flash analysis
- specific heat capacity
- thermal conductivity
- thermal diffusivity
- thermal inertia
- H. D. Baehr and K. Stephan, Wärme- und Stoffübertragung 4. Auflage, Springer, (2004), p 172, ISBN 3-540-40130-X