Wiktionary
isosbestic point
n. (context spectroscopy English) A specific wavelength at which two chemical species have the same molar absorptivity.
Wikipedia
Isosbestic point
In spectroscopy, an isosbestic point is a specific wavelength, wavenumber or frequency at which the total absorbance of a sample does not change during a chemical reaction or a physical change of the sample. The word derives from two Greek words: "iso", meaning "equal", and "sbestos", meaning "extinguishable".