1. n. [Drilling Fluids]

A fluid in which the shear force is not proportional to the shear rate (non-Newtonian) and that requires a finite shear stress to start and maintain flow. Most drilling muds are characterized as either plastic or pseudoplastic fluids.

See: Bingham plastic model,  Herschel–Bulkley fluid,  non-Newtonian fluid,  power-law fluid,  pseudoplastic,  rheology,  shear rate,  shear stress,  viscosity