<prompt> The most common concept of hard and soft substance isRelative resistance to indentationRelative resistance to abrasionRelative resistance to fractureAll of the above  <response>: The most common concept of hard and soft substances is their relative resistance to indentation. Hardness is therefore a measure of the resistance to plastic deformation and is measured as a force per unit area of indentation. Hardness may be broadly defined as the resistance to permanent surface indentation or penetration. 