Yield Strength: 

The stress/strain point at which material behavior changes from elastic to plastic is known as the Yield Point. The amount of stress needed to bring a material to this point is known as its yield strength.  In practice this is difficult to determine, the Offset Yield Point is used in tabulations of material properties. At the Offset Yield Point the sample has deformed plastically to an extent that will leave the material with a permanent strain of 0.002 (0.2%) when it is at zero stress. The stress, at which this occurs, is the Offset Yield Stress.