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 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. 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.