Yield Stress, sy
Index
The yield stress defines the boundary between the elastic and plastic response of a material. Above the yield stress the material will deform plastically, below the yield stress the material response is elastic.

The exact location of this elastic-plastic boundary, P, is hard to determine and an offset yield stress (proof stress), sy, has been defined as that stress which produces a given permenant plastic deformation in the sample. Typically, the permenant strain has the value 0.002.

From: Callister, "Materials Science and Engineering," Wiley (1997)