What property defines a material's ability to resist scratching, indentation, or penetration?

The property that defines a material's ability to resist scratching, indentation, or penetration is hardness. Hardness is a measure of how resistant a material is to localized plastic deformation, which can occur when an object applies pressure to its surface. This property is crucial in manufacturing and material selection, particularly in applications where wear resistance is important, such as in tools, machinery, and structural components.

In contrast, toughness refers to a material's ability to absorb energy and deform plastically without fracturing, making it more about the material's resilience than its ability to resist surface damage. Ductility describes the extent to which a material can be deformed under tensile stress, indicating how much a material can stretch before breaking. Strength typically refers to the ability of a material to withstand an applied load without failure but does not specifically address the surface characteristics that relate to hardness. Thus, hardness is the best choice for defining a material's resistance to scratching and penetration.