In physics, shear stress is a type of internal tension that a body can experience. In ceramics, it is more commonly referred to the MAGNITUDE of the force applied to a fluid (slip, glaze or grit suspension, etc.) to generate movement.

This value is represented on Cartesian diagrams (vertical axis) along with the velocity gradient (a physical quantity related to the fluid's movement speed, on the horizontal axis of the diagram), and the combination of these values provides a general, though partial, picture of some rheological values of the suspension, such as viscosity and yield stress, allowing for proper management.

Newtonian fluids like water and oil, for example, do not change their viscosity value when shear stress varies (whether they are mixed quickly or slowly).

Conversely, plastic fluids exhibit behavior characterized by a specific yield stress, which is a shear stress value below which the fluid behaves like a solid. When the fluid is at rest, it presents an almost solid compactness. If the fluid is set in motion (by applying shear stress), its consistency softens, taking on the characteristics of a liquid. A classic example of a fluid with plastic behavior is custard.

In ceramics, discussing fluids with plastic behavior mainly refers to slips and aqueous grit suspensions.