The stress level where the material starts to strain plastically is termed the yield stress, σy. When a material is stressed by an amount that is less than the materials yield stress it will only undergo elastic (reversible) strain, and no permanent deformation of the material will occur.
The level of stress that corresponds to the yield point is referred to as the yield strength of the material. The yield strength, σy, is a material constant.
It is often difficult to determine the exact point where yielding begins, so in practice a proof stress is used. This is where the stress to give a certain amount of strain (for example 0.2%) is used to define the yield strength (and in this case would be called the 0.2% proof stress).