How you call the constant $\alpha$ within the heat equation in general and in terms of electromagnetism?

The heat equation or diffusion equation does contain a constant $\alpha$.

$$\frac{\partial u}{\partial t} - \alpha \nabla^2 u=0$$

How is it called?

I'm interested in a general name which can be used for different circumstances apart from heat transfer and diffusion of fluids. For example in electromagnetism this constant is related to the skindepth $\delta$ via:

$$\alpha = \frac{\sqrt{2j}}{\delta} = \sqrt{j\omega \kappa \mu}$$

If there is no name, what would you suggest?

• For an electromagnetic wave, this is the propagation constan. – Martin Petrei Aug 1 '14 at 14:36
• @Tinchito thanks! that was already was I was looking for! – thewaywewalk Aug 1 '14 at 14:38
• sure? then I post it as an answer... :) – Martin Petrei Aug 1 '14 at 14:42
• @Tinchito: Well I think it is alright, what I'm doing has nothing really to do with waves, rather effective reluctances in different materials, thats why I never stumbled of this name the whole time. But I think it is appropriate for this case also. – thewaywewalk Aug 1 '14 at 14:45
• I understand. When you find the solution of the Helmholtz equation for the electric field (wave equation), the meaning of this constant is evident, and hence its name. – Martin Petrei Aug 1 '14 at 14:54