## Wikipedia - Numero d'onda (it) Wikipedia - عدد الموجة (ar) wavenumber, $$\sigma$$, $$\tilde{\nu }$$

https://doi.org/10.1351/goldbook.W06664
The reciprocal of the @[email protected], $$\lambda$$, or the number of waves per unit length along the direction of @[email protected] 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 [Terms] [Book]
PAC, 1985, 57, 105. (Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)) on page 109 [Terms] [Paper]
PAC, 1996, 68, 2223. (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2283 [Terms] [Paper]