The shear stress σ 0 or τ 0 at which yielding starts abruptly. Its value depends on the criterion used to determine when yielding occurs.
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)