The stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain is known as:
Yield strength often is a property that represents the stress value at which a small amount (0.1% or 0.2%) of plastic strain has occurred. A value of either 0.1% or 0.2% of the plastic strain is often selected and is referred to as the percent offset. The yield strength is the stress required to produce the particular offset strain (0.1% or 0.2%) that has been chosen. The yield strength for 0.2%  offset  is  greater than that associated with an offset of 0.1%. If yield strength values for two materials tested under the same conditions are to be compared, identical offset values should be used.
Key concept:  Yield strength is defined as the stress at which a material exhibits a specified limiting deviation from proportionality of stress to strain.
Phillips dental materials 12th edition page no 49,57
Proportional limit
Tensile strength
Ultimate strength
Yield strength