Define the term "stress" as it relates to materials.

In the context of materials science and engineering, "stress" is defined as the internal force per unit area within materials. This concept is crucial for understanding how materials respond to external loading conditions. When a force is applied to a material, it creates internal resistive forces that act to maintain equilibrium within the material structure. These internal forces, distributed over a given cross-sectional area, give rise to stress, which is typically measured in pascals (Pa) or pounds per square inch (psi).

Understanding stress is essential because it helps engineers design structures and components that can withstand the forces they encounter during service. Stress analysis informs decisions related to material selection and safety factors in engineering applications, as different materials exhibit varying levels of strength and deformation characteristics under stress.

The other options present different concepts related to materials. Deformation under temperature changes pertains to thermal expansion, rather than stress specifically. The external force applied on a material refers to the applied load but does not account for the internal responses of the material. Elasticity relates to a material’s ability to return to its original shape after deformation, but does not define stress itself. Thus, the correct definition of stress as the internal force per unit area is fundamental for understanding material behavior and structural integrity.