A non-Newtonian fluid is one that exhibits a viscosity that varies as a function of the applied shear rate or shear stress. 

Examples of non-Newtonian materials are shear thinning (pseudoplastic), plastic (exhibiting a Yield StressYield stress is defined as the stress below which no flow occurs; literally behaves like a weak solid at rest and a liquid when yielded.yield stress) and shear thickening (dilatant).

Most formulated materials are non-Newtonian to give beneficial material functionality, such as a high viscosity at rest to promote stability, but then shear thinning to facilitate ease of application. Kriechen (Rheologie)Creep is one of the earliest “controlled stress” rheometer tests that quite literally “creeps” the material, i.e. we measure over a relatively prolonged period the small movement (the creep defined as creep compliance, J) of the sample by applying a small constant stress.Rheology is the study of this non-Newtonian behaviour which results from the “break down” under shear of the “associated” (micro)structure in the sample.