Hardness of a material is a measure of its resistance to localized plastic deformation, such as an indentation or a scratch, induced mechanically by pressing or abrasion. It is the ability of a material to withstand force without deformation, scratching, penetration, and indentation. Hardness varies among different materials, and it depends on several factors, such as plasticity, elastic stiffness, ductility, toughness, strain, strength, viscoelasticity, and viscosity. Hardness can be measured in different ways, such as scratch hardness, rebound hardness, and indentation hardness. Scratch hardness is the ability of a material to resist scratching, rebound hardness is the ability of a material to resist deformation under a bouncing ball, and indentation hardness is the ability of a material to resist deformation under a static load. Hardness is not a fundamental material property, and it should be viewed in context with other properties such as strength, elasticity, and ductility to understand how these properties relate to each other.