The study of systems by the electromagnetic radiation with which they interact or that they produce. By convention, spectroscopic properties are expressed in terms of wavelength
in vacuo, not the wavelength within the medium being studied.
Source:
PAC, 1985, 57, 105. 'Names, symbols, definitions and units of quantities in optical spectroscopy (Recommendations 1984)' on page 107 (https://doi.org/10.1351/pac198557010105)