Denoted $\mu$ typically. Defined as the mean mass per particle, in units of ([[Units & Conversions#Atomic Mass Unit ($amu$)|amu]] $\simeq$ the mass of a hydrogen atom). Start with mean mass per particle$\bar{m} = \frac{\sum_{\rm{species}\,j} n_{j,\rm{ion}}m_{j,\rm{ion}} + n_{e}m_{e} }{\sum_{\rm{species}\,j} n_{j,\rm{ion}} + n_e} \simeq \frac{\sum_{\rm{species}\,j} n_{j,\rm{ion}}m_{j,\rm{ion}}}{\sum_{\rm{species}\,j} n_{j,\rm{ion}} + n_e}$and then $\mu = \frac{\bar{m}}{1\,\pu{amu}} = \frac{\sum_{\rm{species}\,j} n_{j,\rm{ion}}A_j}{\sum_{\rm{species}\,j} n_{j,\rm{ion}} + n_e}$ where $A_j$ is the atomic number of species $j$ (ie the number of protons and neutrons).
**Some examples**:
| Species | $\mu$ | Comment |
|:------------------------------------------------------------------ |:-------------------------:|:-------------------------------------------------------------:|
| [[Interstellar Medium#Atomic Hydrogen\|Atomic (Neutral) Hydrogen]] | $\mu \simeq 1$ | |
| [[Interstellar Medium#Ionized Hydrogen]] | $\mu \simeq \tfrac{1}{2}$ | we doubled the number of particles but didn't change the mass |
| [[Interstellar Medium#Molecular Hydrogen]] | $\mu \simeq 2$ | particle number invariant but mass doubles |
| Atomic (neutral) Helium | $\mu \simeq 4$ | |
| Completely Ionized Helium | $\mu \simeq \tfrac{4}{3}$ | |
- [Reference1](https://faculty.fiu.edu/~vanhamme/ast3213/mu.pdf)
- [Reference2](http://astronomy.nmsu.edu/jasonj/565/docs/09_03.pdf)