Elongation is a term for increasing the length of a body. This change in length can be caused by a mechanical force or temperature. The opposite of stretching is compression, in which the length of the body is reduced.
If a body is stretched, there is a compression transverse to the direction of force so that the volume of the body remains approximately the same (transverse contraction).
Stretching a body therefore reduces its cross-section. The elongation is calculated by relating the change in length to the initial length.