Final answer:
The ratio between the ultimate strength and the allowable stress is defined as the Factor of safety. Factor of safety is a measure used in engineering to determine the safety margin of a structure or material.
Step-by-step explanation:
The ratio between the ultimate strength and the allowable stress is defined as D. Factor of safety. Factor of safety is a measure used in engineering to determine the safety margin of a structure or material. It is calculated by dividing the ultimate strength of the material by the maximum allowable stress.