a mathematical formula relating subjective experience to changes in physical stimulus intensity: Specifically, the sensation experienced is proportional to the logarithm of the stimulus magnitude. It is derived from Weber’s law and expressed as Ψ = k logS, where Ψ is the sensation, k is a constant, and S is the physical intensity of the stimulus. Also called Weber–Fechner law. See also Stevens law. [Gustav Theodor Fechner]