wavelength, λ

https://dx.doi.org/10.1351/goldbook.W06659
Distance in the direction of propagation of a periodic wave between two successive points where at a given time the phase is the same.
Source:
ISO 31-5: 1992 (Quantities and Units - Part 5: Electricity and Magnetism.)
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 998
See also:
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