Final answer:
The slope of the line in the elastic portion of the stress-strain diagram is referred to as the elastic modulus or Young's modulus.
Step-by-step explanation:
In the elastic portion of the stress-strain diagram, the slope of the line is referred to as the elastic modulus or the Young's modulus. It represents the measure of stiffness of a material and relates the stress applied to a material to the resulting strain. The elastic modulus is calculated by dividing the stress by the strain.