The measure of resistance to indentation or scratching in a material is called:

The measure of resistance to indentation or scratching in a material is referred to as hardness. Hardness indicates how well a material can withstand wear, pressure, or erosion, making it an important property in materials selection, especially in HVAC applications where components may be subjected to various mechanical stresses.

Ductility relates to a material's ability to deform under tensile stress, allowing it to be stretched into wires without breaking, which is not directly related to resistance to indentation or scratching. Toughness measures a material's ability to absorb energy and plastically deform without fracturing, focusing on impact resistance rather than surface hardness. Strength is a general term that describes a material's ability to withstand an applied load without failure, but it does not specifically address the resistance to scratching or indentation.

Given these definitions, hardness stands out as the most relevant term for describing resistance to surface deformation when pressure is applied, making it the correct answer in this context.