differential viscosity

The derivative η Δ of stress with respect to the rate of shear at a given shear rate.
Source:
PAC, 1979, 51, 1213 (Manual of symbols and terminology for physicochemical quantities and units. Appendix II: Definitions, terminology and symbols in colloid and surface chemistry. Part 1.13. Selected definitions, terminology and symbols for rheological properties) on page 1217