(1) Capacity of a metal to absorb energy and deform plastically before fracturing. Usually measured in a notch impact test, high impact values indicating high toughness, but the area under the stress-strain curve in tensile testing is also a measure of toughness. (2) The ability of a metal to rapidly distribute within itself both the stress and strain caused by a suddenly applied load, or more simply expressed, the ability of a material to withstand shock loading. It is the exact opposite of "brittleness" which carries the implication of sudden failure. A brittle material has little resistance to failure once the elastic limit has been reached.