Boundary condition of the third kind (Fourier's)
A condition which consists in prescribing the dependence of the heat flux due to conduction on the surface of a body on the temperatures both of the surface of the body and of the surrounding medium.
Comments. 1. If the heat flux over the surface of a body is proportional to the temperature difference between the surface and the surrounding medium, the boundary conditions of the third kind are called "linear boundary conditions of the third kind"; in the opposite case they are called "nonlinear boundary conditions of the third kind."
2. Other forms of boundary conditions are also encountered in practice. At the present time no firmly established terminology exists and, therefore, they are not included in the present recommendations.