What is defined as internal resistance to an external load?

The term that refers to internal resistance to an external load is stress. In the context of materials and engineering, stress is defined as the internal force per unit area within a material that arises from externally applied forces, uneven heating, or permanent deformation, which can lead to the material's failure.

When an external load is applied to a material, the internal structure of that material interacts with that load through internal resistance. Stress quantifies this resistance, indicating how much internal force is generated in response to the load. Understanding stress is critical for ensuring that materials can withstand forces without yielding or breaking.

The other choices pertain to different aspects of material properties. Strain is the deformation of a material in response to stress; hardness measures a material's resistance to deformation, particularly indentation; and tensile stress specifically refers to stress that occurs when a material is subjected to pulling or stretching forces. Each of these terms describes different phenomena, but stress is the correct term for the internal resistance under external loads.