The critical stress at which material will start to flow is called the yield strength. It is the stress at which a material begins to deform permanently, or plastically. This is in contrast to elastic deformation, which is temporary and the material returns to its original shape once the stress is removed.

The yield strength can be determined by performing a tensile test, in which a specimen of the material is stretched until it breaks. The stress-strain curve from this test shows the yield point as the point at which the material's deformation becomes plastic.

The yield strength is an important property for materials used in engineering applications, as it determines the maximum stress that can be applied before the material begins to deform permanently.