The reciprocal of the wavelength, , or the number of waves per unit length along the direction of propagation. The SI unit is , but a commonly used unit is . Symbols in a vacuum, in a medium.
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2283
PAC, 1985, 57, 105 (Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)) on page 109