electron number density unit: $\pu{m-3}$

https://doi.org/10.1351/goldbook.08470

Number of free electrons in vapour phase per volume unit.

Note:
SI unit: $\pu{m-3}$.

Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 717 (https://doi.org/10.1515/pac-2019-0203)