The derivative ηΔ of stress with respect to the rate of shear at a given shear rate.
Source:
PAC, 1979, 51, 1213. 'Terminology and Symbols in Colloid and Surface Chemistry Part 1.13. Definitions, Terminology and Symbols for Rheological Properties' on page 1217 (https://doi.org/10.1351/pac197951051213)