symbol: $\lambda_{0}$, units: $\pu{m}$, $\pu{\upmu\!m}$, $\pu{nm}$
synonym: vacuum wavelength
https://doi.org/10.1351/goldbook.08300
Distance travelled by a wave of electromagnetic radiation in one cycle in vacuum.
Note: SI unit: \(\pu{m}\). Common unit: \(\pu{nm}\) or \(\pu{\upmu\!m}\).