[1] The term is commonly used in absorption spectroscopy, corresponding to a wavelength of maximum absorption (electromagnetic radiation) ranging around 400 nm in the blue region.
The Soret band arises primarily due to an electron dipole movement that allows π-π* transitions; most common in porphyrin compounds.
For example, the "Soret peak" is used to describe the absorption of vividly-pigmented heme-containing moieties, such as various cytochromes.
For example, the cytochromes P450, a diverse class of monooxygenase enzymes, exhibit a Soret peak at 450 nm in their reduced form when saturated with carbon monoxide.
If the P450 has been denatured, for example due to bad handling, the peak shifts to 420 nm.