An error which arises when the absorbance of some samples changes with time, e.g. as a result of photochemical reaction, formation of aggregates or adsorption on the cell wall.
Source:
PAC, 1988, 60, 1449. 'Nomenclature, symbols, units and their usage in spectrochemical analysis - VII. Molecular absorption spectroscopy, ultraviolet and visible (UV/VIS) (Recommendations 1988)' on page 1457 (https://doi.org/10.1351/pac198860091449)