Rheology is the science for flow and deformation of matters. Although rheology is closely related to tribology as seen in the Stribeck diagram, it must be separately considered. For instance, rheology has been developed for linear viscoelasticity owing to the linear response theory, whereas the material response discussed in tribology is in general under highly non-equilibrium. Besides, even under fast and large deformations, rheological measurements are usually performed under laminar flows and non-slip boundary conditions, which are far from the tribological conditions.