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The **magnetic diffusivity** is a parameter in plasma physics which appears in the magnetic Reynolds number. It has SI units of m²/s and is defined as:^{[1]}

- ,

while in Gaussian units it can be defined as

- .

In the above, is the permeability of free space, is the speed of light, and is the electrical conductivity of the material in question. In case of a plasma, this is the conductivity due to Coulomb or neutral collisions: , where

- is the electron density.
- is the electron charge.
- is the electron mass.
- is the collision frequency.

**^**W. Baumjohann and R. A. Treumann,*Basic Space Plasma Physics*, Imperial College Press, 1997.