wavenumber $$\sigma$$, $$\tilde{\nu }$$

https://doi.org/10.1351/goldbook.W06664
The reciprocal of the wavelength, $$\lambda$$, or the number of waves per unit length along the direction of propagation. The SI unit is $$\text{m}^{-1}$$, but a commonly used unit is $$\text{cm}^{-1}$$. Symbols $$\overset{˜}{\nu }$$ in a vacuum, $$\sigma$$ in a medium.
Sources:
Green Book,2nd ed., p. 30 (https://www.cintegrado.com.br/site/documentos/green_book_2ed.pdf)
PAC, 1985, 57, 105. 'Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)' on page 109 (https://doi.org/10.1351/pac198557010105)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2283 (https://doi.org/10.1351/pac199668122223)